Counter-intuitive thought on temp & visc.

[mirou]

Hello, I just got done changing the oil in my '91 camry and because it's summer I used Superflo 10w40(3oz auto-rx, of course), but then I started thinking(thats always trouble). I used a thicker oil because I expect high ambiant temps and a thicker oil won't thin as much. Maybe this is the wrong approach, a thicker oil will also raise the temp of my oil and engine, and it will be doing it during the hottest months. Now if you take one of the good stable 5W30 oils we have seen tested here and you use it instead of the thicker oil you will keep your engine cooler and still be confident of a strong film strength. Could you bring yourself to switch from 10w30 in spring to a 5w30 or even a 5w20 in preparation for a summer heatwave? You could also add lube control or #132. Is this crazy? It seems these light oils are really solid and could easily replace most thicker oils. Joe

 

[moribundman]

My instinct tells me to use a thicker oil like 15W-50 if the ambient is high and if the engine is running hot (due to high speed driving/towing a trailer, uphill driving). If I look at the oil chart for my car, thicker oil is recommended for higher ambient temperatures. I'm reluctent to believe that, for example, a 0W-40 offers the same protection against viscosity breakdown as a 15W-50 oil.

 

[mirou]

Thats exactly what I mean, it is drilled into us that we need to go thick in summer. While it is probably true for towing a heavy load or any severe load on the engine, if your just driving around with the a/c on, a thicker oil will make your car hotter than it would be with 5w30. As Patman says, the cooling system should keep your engine at the same temp in summer as in winter. I am starting to think that with the advent of really good thin oils (Schaeffers, M1, Amsoil,RL) we should use the thinnest oil that will do the job. Could Ford and honda have the right idea? A 5w20 oil doesn't have to cover a very wide spread so it might not be in the same boat as a 0w. Joe

 

[mikemc]

Thinner oils flow at a higher volume than thicker oils at the same temp. and transfer heat more quickly. The oil certainly needs to be strong enough to maintain a film of protection without shearing under heat or stress. A well built 10w30 in summer and 5w30 in winter seem about perfect to me. I still feel uneasy about 5w20, but UOA is generally not supporting my fears.

 

[moribundman]

Let's face it, the worst we face in the US is standing in a jam, engine idling, A/C on, it's 100 degree F. How does that compare to driving 100 or more MPH for an hour or two on the Autobahn? Idling under adverse conditions can't be better than running an engine under load at high RPM. Sure, I'm in California, and I see below freezing temperatures maybe on three days during a year, but does that mean I need 0W-40 oil? Hardly. I don't drive my car like granny drives her Buick to the supermarket. If 15W-50 makes no sense, why does Mobil still offer it? Look what Mobils says about 0W-40 and 15W-50. It says practically the same thing for both oils:

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Mobil 1 0W-40 is recommended for all types of modern vehicles, especially high-performance turbo-charged, supercharged gasoline and diesel multi-valve fuel injected engines found in passenger cars, SUVs, light vans and trucks. * Mobil 1 0W-40 is especially suitable for extreme conditions, where conventional oil often cannot perform." Mobil 1 15W-50 is recommended for all types of vehicles, suitable for use in high-performance turbo-charged, supercharged gasoline and diesel multi-valve fuel injected engines found in passenger cars, light vans and trucks. * Mobil 1 15W-50 is suitable extreme engine stress conditions, where conventional oil often cannot perform.

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Sounds the same to me, apart from the fact that 15W-50 is not recommended for SUVs - it's all hogwash!

 

[moribundman]

mikemc, yes, thinner oil flows better and dissipates heat better (Is that true at extremely high temperatures, or only at normal oil temps?), but thicker oil provides higher oil pressure, and I think under extreme conditions I'd rather not risk the thin oil crapping out on my engine. Why is is that BMW recommends 10W-60 oil? I don't think BMW engines have wider-than-average clearances or tolerances compared to other modern motors.

 

[Jay]

BMW recommends 10w-60 oil in one of their engines as a bandaid for an engineering flaw in the bottom end design. Why not just use the viscosity that the owner's manual recommends?

 

[moribundman]

Here's what my manual recommends: Anything goes! http://pictureposter.audiworld.com/50038/oil_chart.jpg Click the above link and reload if pic doesn't show up.

 

[Schmoe]

The 5W-20's still make me nervous. My general understanding from this board is 1. if your engine is using oil, move up in viscosity. 2. Winter months (cold like in your neck of the woods) go a little thinner 3. summer you want to go a little thicker. All my vehicles get 5W-30's in winter and then 10W-30 in summer, regardless of what the manufacturer says. Just posted my UOA on Quak. State 5W-20, Blackstone said it was getting a little thin with 3345 miles on it. Compared to others how have posted various 10W-30's, their oils held up fine well over that amount of miles. I don't think I've ever seen or heard of a problem, at least on this board, from any member having oil temperature related problems. With your engine and age of the car, I think a 40 would be a little high, unless your losing oil. [ June 19, 2003, 01:12 PM: Message edited by: Schmoe ]

 

[moribundman]

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What is that pic of??

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Click on the pic and choose "View Picture" - then it will load.

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The problem with your understanding is that the primary function of oil is NOT to "cool" your engine. That's the cooling system's job. That said, your motor oil's job is to lubricate...and a thicker grade oil can withstand higher temp. for longer duration with less breakdown, volatility, etc. So, if you're worried about oil temps., you get an oil cooler, not thinner oil. eg. NASCAR.

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MY comprehension is just fine. Thank you very much. Better read again. Nowhere did I say that oil is supposed to "cool" my engine. I said that oil will break down thermally under adverse conditions. High ambient temperatures are however contributing to elevated oil temperatures and low oil pressure, as evidenced by prolonged highspeed driving, at which the oil reaches easily 230 degrees, while the coolant stays below the boiling point (I've never seen my coolant go beyond the boiling point, while the oil can get very hot). Look at the chart (if the picture will work), and see for yourself that a thicker oil is recommended for higher ambient temps. The question is, at what point becomes the advantage of a thinner oil and its ability to dissipate heat better less important than protection from thermal breakdown?

 

[MolaKule]

Since the coolant is under pressure, I doubt it will boil, whereas the oil is at ambient pressure and will boil if heated high enough. One of the reasons why I use a narrow viscosity oil (10W30) is because of the very question you ask. Since most physical optimization is a balancing act, the 10W30 is the best compromise between competing requirements.

 

[moribundman]

With the coolant again... Sure, "physical optimization is a balancing act" - and that's exactly my point. At the risk of sounding like a parrot, this is what I'd like to know: At what point (regarding ambient temperature and engine load) becomes the advantage of a thinner oil and its ability to dissipate heat better less important than protection from thermal breakdown?

 

[MolaKule]

Well, this is one of the answers I hope to realize from the viscosity vs. clearances study. Initial results show that for higher rpms (at the same load) requires a lower viscosity oil. So this aspect takes care of itself. At higher rpm, you're going to get more flow, even though the oil is hotter - but since it is hotter, it is going to have a lower viscosity anyway and carry heat away at a faster rate. As far as thermal breakdown, I will assume you mean that oil will heat up to the point that it thins too much for hydrodynamic cushion (protection). Well, that's what barrier additives are for. If the viscosity index (VI) of the oil is such that that say a 10W30 will only thin to an SAE 15 or 20 weight at the higher temps, then you will still have hydrodynamic protection backed up by barrier adds. I opt for flow and cooling over higher pressure and thicker viscosities.

 

[Dr. T]

If you're not asking about the ability of oil to "cool your engine" then, why the question about it's ability to dissipate heat? Although I can see some reasoning here...it's faltered because...where is this heat dissipation taking place? Just as your cooling system and coolant contained within would be USELESS without a radiator to "dissipate" the heat, oil has nowhere to readily dissipate heat unless you get an oil cooler.

 

[MolaKule]

Somewhere here I posted the fugures for heat dissipation percentages for both gas and diesel engines. The oil cools down by radiation, convection, and conduction. Some of the oil's heat has to be dissipated by the coolant heat rejection load. Some of the oil's heat is removed by convection, aka, the air stream hitting the pan. About 1% of the heat is removed by radiation, which is somewhat insignificant. In my view, if the cooling system is rejecting heat properly, the oil should not thin to a catastrophic viscosity. [ June 19, 2003, 03:46 PM: Message edited by: MolaKule ]

 

[moribundman]

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If you're not asking about the ability of oil to "cool your engine" then, why the question about it's ability to dissipate heat? Although I can see some reasoning here...it's faltered because...where is this heat dissipation taking place? Just as your cooling system and coolant contained within would be USELESS without a radiator to "dissipate" the heat, oil has nowhere to readily dissipate heat unless you get an oil cooler.

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Actually, since thinner oil moves faster way from hot engine parts (It's not like an engine doesn't have especially hot spots - like the COMBUSTION CHAMBER!) it WILL dissipate heat faster than a thick oil. I wouldn't exactly call that "cooling," but I don't want to get into semantics here. As for an oil cooler, my car has an oil cooler. But that's again besides he point. If you're driving in the friggin' desert, that oil cooler won't do much good. As I've said before, I have in the past (not with my current car) suffered loss of oil pressure after prolonged high-speed driving in the summer (without the coolant getting hotter than normally - and that car did have an oil cooler, too), and I was advised to run a thicker oil, which did remedy the low-oil-pressure problem.

 

[moribundman]

The question still stands: At what point (regarding ambient temperature and engine load) becomes the advantage of a thinner oil and its ability to dissipate heat better less important than protection from thermal breakdown? I am pretty sure that a thicker oil can be of advantage under extreme conditions. Once again, I'm not talking about cruising around at 55mph at 65 degrees!

 

[Dr. T]

My answer would be: "when oil consumption occurs beyond normal" it's ability to dissipate heat is negated by thermal instability/breakdown. I believe that ideally there should be zero consumption between oil change intervals because any amount of consumption in a non-worn out engine means the oil is volatilizing and the volatile particles and by-products are coating the engine and componenets contained therein ultimately contributing to sludge and varnish formation.

 

[MolaKule]

I am assuming here that the duscussion is about daily drivers, and one has to define the conditions of operation when one asks these types of questions. Sure, thicker oil will give higher pressure, because the thicker oil is moving slower due to its higher viscosity. IF the oil flow is slowed or interrupted for some reason, then a higher viscosity oil may prolong catastrophic failure. Even with a high viscosity oil, I have yet to see anyone finish a race without some oil flow cooling the bearings and rings. In my view, thicker oils are only applicable for extreme situations and where fuel dilution from exotic fuels or rich mixtures would be a problem, or where you are running a totally air-cooled engine, and the major heat rejection (cooling) is via the oil. You have to consider the application verses the viscosity. That's the tradeoff. "At what point (regarding ambient temperature and engine load) becomes the advantage of a thinner oil and its ability to dissipate heat better - less important than protection from thermal breakdown?" And again I ask the question, what do you mean by "thermal breakdown" of the oil? If you mean the thinning of the oil to a viscosity such that the hydrodynamic film is lost, then barrier additives kick in. The barrier additives will only prolong the catastrophic failure until the heat builds up and meltdown occurs. So my answer is that assuming your oil pump and coolant systems are functioning properly, the thinner oil (within reason) is the better choice for average driving, since the thinner oil will cool better, and you will not see viscosity thinning to the point of loss of hydrodynamic film. If your daily driver sees oil temps averaging above 250 F, then you had better change your driving style or investigate why those temps are so high. If this doesn't answer the question, then maybe an example of the conditions you speak of will clarify.

 

[moribundman]

I'll gladly give you a concrete example: Having driven on the Autobahn at full speed (200km/h=ca125mph) for sometimes an hour or longer, I have seen my oil thin to the point where the warninng light and buzzer have come on - but I've seen that happen only with THIN oil, and NEVER with thicker oil. And in neither case did the coolant get excessively hot. German cars are made for long distance high-speed driving. I'm not driving Autobahn in the US, but I do suffer traffic jams on Highway 101 in the middle of the summer (at over 100 degr F), I'm sort of worried using a thin oil. In a jam, idling for 10 minutes or longer, is the only time I ever seem the coolant temperature rise. However, with 0W-40 oil I do get higher oil temperatures than I used to get with Syntec 5W-50. And that's on hot days when driving the car hard in the twisties. That's why I'm wondering if M1 15W-50 might be more suitable than 0W-40 under those conditions. My suspicion is affirmed by the oil chart that Audi provides for my car. The oil they list for ambient temps above 100 degr F is actually 20W-50. Now, the car and the chart are almost 7 years old and the capacity of oil has progressed, but to what point? Have we reached a point where a thin oil is on par with a thick oil? I hope that clarifies it?