best overall viscosity

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May 30, 2002
I would tend to agree. If you live in -0f temps in winter the 5w-30 is best. Although, if i did not tow my boat and work my engine hard, i would use 5w-30.
For most new cars, it is my belief that 10w30 is the best viscosity for all weather other than the cold winter time. I still think 5w30 was simply adopted because of fuel economy, and the added VI improvers in it are a detriment to it's overall performance.
Bob, I don't think you can ask a blanket statement like that...kinda like asking which are the best size of condoms out there....
. eg. BMW M5's spec. out a Castrol 10W-60 made in the U.K.. This is the best visc. out there for this car...other cars are different. Why? Because different engines have different viscosity requirements as illustrated above. That is why there's an owner's manual. That's what the engineers are spec. out the proper visc. for the designed engine parameters within a given operating temp. range. Hope this helps.
It makes you wonder why a 10W60 is specd?

Could it be that the clearances are
getting larger in order to decrease friction and to accomodate more wear material,
for longer intervals without excessive wear?

Well, obviously it's a Euro. spec. oil...the said engine is a high revving multivalve VANOS wonder that probably requires this oil to ensure a safety barrier due to shearing that must occur under such strenuous conditions. Ditch the fuel econo. BS in this case...
With a 10w60 you are looking at an oil that almost doesn't thin out. Somewhere around 32 to 35 cSt at 40C and still about 25 cSt at 100C. I see better proteccion in turbos and hot areas, but you are right, it will cost a little in fuel economy.
Penrite just released full syn 5W-60 here. Interesting but seems a very wide range to cover. Good for high heat/load maybe but how long will it last? Would like to see analysis figures on it thats fer sure!
best weight is typcially the thickest reccomended by the manufacturer. Most manufacturers will say 2 or 3 weights are fine depending onconditions (temperature, load on engine, etc..)

1992+ Corvettes/Trans Ams/Camaros have all been spec'd for 5W30 in winter, and 10W30 in summer or in southern states with hot temps (like CA, AZ, TX).

guys who run these cars HARD at a road course (1hour at 5000 to 6000RPMS
) in elevated temperatures demand synthetics, and typically you hear of guys running 20W50. The only catch is that you HAVE to run it hot and hard to make this ok on your bearings. If the engine doens't heat the oil enough you risk losing your oil film and having nasty engine wear.

What it comes down to is finding a oil that is at a viscocity that will match the engine bearing tolerances for the normal starting and operating temperatures.

Oil film is everything.

(I've been told that 4000+hp topfuel dragsters typically run a 70 to 90 weight oil since any thinner will literally be pushed out and beat into a froth while the engine runs metal on metal!!)

[ June 07, 2002, 12:31 AM: Message edited by: Steve in Seattle ]
is it the general consensus that 10w-30 is the
best "overall" weight motor oil?
Alright, this is going to sound somewhat unconventional, but it might make for some interesting conversation:

Why not a straight grade 30 in full synthetic? The Pour Point is -38F and there are no polymers to shear. Is there any reason why this could not be used in higher mileage passenger vehicles, even in winter?
GW, many of Red Line oils (and other synthetics) are actually a straight weight but their synthetic nature of flowing well at the extremes makes them act like multi-vis dino oils even though some have no pour point depressants nor viscosity improvers.

Take a look at Red Line's racing weights on their site:
Yeah well, on that note...most synth. even 15-50's will pour at -45F. How lives in these conditions? So why wouldn't you be able to use this grade all year round? Why putz around with 5-30 or even worse 5-20? I think the like for 0-30's, etc....

Obviously the manuf. specs are geared towards conv. oils that would be wax at those temps...

Yes I see what you mean. The only thing with these Redline straight-grade oils is that you will not get any detergent additives - this may be fine for race applications, but not for the average driver.

I was referring to Amsoil's SAE 30 Diesel Oil in my post - what are your thoughts on this?
Royal Purple's race oils have a TBN of 11, but I'm not sure on their other technical specs. I do know that they are single weight oils but RP shows their corresponding multi-weight equivalent. I'd love to see data on these oils to see if they'd hold up for everyday use. RP techs claim they will, but of course they are biased.
Here are some specs. for Amsoil SAE 30. It would seem that this oil does jump through the multi-viscosity hoops in the same way that Redline's single weight race oils do.

At 40C, this oil's kinematic viscosity falls between a 10W30 and a 10W40 (petroleum) at the same temperature.

Kinematic Viscosity @ 100°C, cSt 11.5
Kinematic Viscosity @ 40°C, cSt 85.4
Viscosity Index 124

Pour Point °C (°F) (ASTM D 97) -38 (-36)
Flash Point °C (°F)(ASTM D 92) 232 (450)
Fire Point °C (°F) (ASTM D 92) 252 (486)
although 124 is a good VI for a straight grade oil, any straight grade oil will give you harder starting, more wear on upper engine parts at starting, poorer gas mileage, and LESS film thickness at operating temperatures. Check this for a comparison of SAE 40 and SAE 15w40 viscosities at different temperatures.

Thanks for your input. I know that I may have an overly-simplistic view on this is it that this straight grade will have such an effect on cold starting wear etc. when its viscosity is lower than what a 10W40 is @ 40C?

It is true that 40C is not a cold start, but then, if you compare pour points, this oil is same as a petroleum 10W40.

I would also like to hear your thoughts on film strength vs. that of a multi. I was not able to view the page that you posted a link to. Why is it that Detroit Diesel recommends only straight grade oils for their hard-working high-revving 2-cycle diesels? It may be more to do with shear but,I have one of these engines that I perform oil analysis on and I can say that its wear rates are very low using SAE 40 petroleum.
SAE 40 is "thicker" (higher viscosity) at 40C than 15w40. It is after 100C that the lines cross and the 15w40 becomes "thicker".
The 40 weight is normally around 140 cSt at 40 C., whereas a 15w40 is normally around 105 or so. This varies slightly with the formulation, but are general numbers. I've seen single grades with a 120 VI, but that is more like a multigrade with imporved pour point that they are not defining the low end.
The higher the VI, the more horizontal the viscosity line on the graph. If anyone made a poylglycol 15w40 (VI's run in the 250's) you would have a real nice and even viscosity curve. But you would pay double or triple what you pay for PAO, and I don't know what their reaction would be on other surfaces or in engines. They are normally used in industrial gearboxes for lifetime fills.
Correct me if I'm wrong but multi-viscosity oils started with dino which had to use a lot of polymer-based viscosity improver to make a thin mineral oil act like a thicker oil at operating temperature.

Along come some of the better synthetic compounds (PAOs & esters) which start off thinner but maintain their film strength and viscosity even as the temps climb.

This goes along with what Widman is saying, I think.

Therefore, synthetics act a lot like multi-viscosity oils without any viscosity improver at all. Most 'real' synthetic 10W30s habve no viscosity improver, for example. And, when they want to be an extreme spread oil, they use a lot less V.I. than a dino oil would.

Now, does this mean that some real synthetics (not just Red Line's race oils) are essentially straight weights? I don't know enough about the definitions to make this sort of proclamation.

Since Red Line's street oils are essentially their race oils with a different additive package, I would guess that the basic properties they claim for their race oils apply to their street oils as well.

So, why not market their street oils as straight grades? Because it would just confuse the motoring public which use multi-vis oils in all modern road cars. Racers, on the other hand, have historically used straight weights so the labeling (dual-labeling, actually) makes more sense for them.

To sum all my rambling up, I believe the difference between the two types is mostly marketing/labeling.

[ June 09, 2002, 07:56 PM: Message edited by: Bror Jace ]
This, Bror Jace, is exactly what I am thinking. What Widman (no offence Widman) is saying about increased start-up wear may well be a generalization which might hold more truth for petroleum oils.

There is alot of talk about straight grades vs. multis in aviation circles - many are looking to an SAE Paper (951035) to guide them in there decision to not use multis. Four engineers, two from Mercedes and two from Shell showed that, in a Mercedes 4-cyl. diesel, straight-grade 30 produced less cylinder wear than both 10W30 and SAE 10 oils. All three of these oils were automotive oils. In any case, this says nothing about valve-train wear and start-up wear.

There is an article about this here:
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