0w-40 in a car that likes 15w-40?

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Staff member
Dec 14, 2002
New Jersey
Yes, good point... what is with that? How is it that its kinematic viscosity at 40 is in the range between what the 10w- and 15w- oils are, yet it still classifies as a 0w- oil? Also, the cold cranking value in centipoise is higher than the 0,5 and 10w-30 oils (oddly it is lower than the 0w-20, but the 0w-20 has lower kinnematic viscosity at 40c) This makes no sense to me, although I am a bit rusty on my rheologcal studies (last time I did complex fluids was a few years ago). This oil is looking better and better in this application.
This is why I frequent this forum... to learn.
Thanks very much,

I completely agree with Patman that you shouldn't worry about the lower number much. At the same time, the viscosity at 40C doesn't tell you much either...it's just a passing phase in the warm-up of the engine...soon to reach 100+C.

Mobil's goal (as always) is to 'sell' pours at -100 or whatever...this is why they sell 0-20, 0-40, etc..

I've used the 0-40 in my BMW. Made too many mechanical sounds and consumed it pretty good, so I switched back to theri 15-50. I know what you're saying re: sluggishness. I now currently use Syntec 5-50. My bimmer engine was seriously awoken. This is what I'd recommend...and then tell us how the puppy drives. Don't worry about the 50 weight...it's lots thinner than the 15-50 and the engine will be quieter and quicker.
I have a 1997 BMW 318i with the M44 engine (as you know this is not the same motor). The car runs fine and has consumed no oil in the past 1,000 or so miles since I put that oil in to replace the Pennzoil 15w-40 that I was using before. I noticed that the car starts faster in cold weather. Cold being anything less than about 40 F.

How many miles does your BMW have? If it does not have too many miles and does not burn any oil than I think the M1 0w-40 would be fine otherwise a good oil to try would be Castrol Syntec 5w-50 as Dr. T has suggested. This is a better product than the Castrol Syntec 5w-30 and 10w-30. The Castrol Syntec 5w-50 and 10w-40 have the ACEA A3/B3 rating. A3/B3 means the high viscosity oil must stay in grade during the ACEA tests. You may have noticed that the Castrol Syntec 5w-30 and 10w-30 do not meet the low viscosity stay in grade rating of A5/B5. The A stands for gas engine and B for diesel engine oil in the ACEA ratings for motor oil. I can only remember seeing one UOA on Castrol Syntec 5w-50 but it had low wear in an Old Chevy Caprice Police car. I do think that it may prove to be better for your application than the M1 15w-50. Just how many cars need an oil that thick these days (15w-50)? While I do think Castrol Syntec 5w-50 and 10w-40 are probably very good oil I would never use Castrol Syntec 5w-30 and 10w-30 because they only meet the weak ACEA A1/B1 ratings.

You may also want to consider Amsoil 10w-40.

I forgot to mention that when I drive the BMW (it's really my wifes car) I just drive the hell out of it and it still does not use any oil that I can tell. When my wife drives it the car does not get treated as badly.

[ May 22, 2003, 03:46 PM: Message edited by: Sin City ]
I think a little heavier oil at cold start up is a good thing. Engine tollerances are larger, and heavier oil keeps things quiet until the pieces mate together when heated up. By that time the oil will have thinned to the correct hot viscosity. Also keeps oil burning to a minimum until the engine is warmed up.
BTW when I'm at Autozone, I shake the oil bottles just to get a feel of how thick the oil is at room temperature. The stuff I'm using now 15w40 Royal Purple is very thick in the bottle at room temps compared to other brands.

Completely agree..do not use the 5-30 or 10-30. The other 2 are great oils. We don't have the 10-40 in Canada...pretty much in any synthetic or blend.

The car is a 1991 BMW 318i (M42 DOHC 4-cyl engine, 6700+RPM redline). I want to change to synthetic, for better engine protection, and perhaps better mileage as well. The owners manual specs 15w-40, 50 or 20w-50 at all temps above -4. I think that going with the 50 end of viscosity is a bit much for this engine, as the car isnt as fast or feels as powerful as my girlfriends equal displacement, equal power integra. I dont want to rob any more power by having something overly viscous, and I think that a 40wt oil is about right. My concern is the lower number. For most all temps, 15 or 20 as the lower cold number is reccomended. At temps up to 50 or 60, you can use a 10 or 5w-30, but were getting into the summertime soon. There is also the ability to use 'specialty oils' which I assume are synthetics, at all range of temperatures.

I feel that Id really like to try the mobil1 0w-40 oil in this car, but Im worried about the 0 vs the reccomended 15 number as the cold weight. I suppose there are some 15w-40 synthetics, such as amsoil, and there is always delvac 1, but I like the energy conserving status of the M1, as well as the very robust specs on the oil (BMW longlife, MB229.5, etc). I have run 15w-50 M1 in there before, but the fuel economy is actually poorer than when using Pennzoil Longlife 15w-40 (which has a nice shot of moly, and not too bad of a base oil).

Should I worry about using the 0w-40 in this car, given the reccomended weights? The car is mostly used around town, usually not driven more than 10 miles and then turned off (and it really cools quickly according to the water temp gauge, plus it has a finned oil pan so that must cool quick as well). I wouyld assume that the 0wt would be good for lubricating quickly once it is started, but I worry about its abilitites in lubricating under load shortly after starting. I never drive the car very hard, maybe over 3000 rpm once in a while (highway cruising is always over 3000, but at that point the oil should be at its proper 40 wt visc.), so with light driving, maybe a 0 wt would be ok?

Any comments/suggestions as to what to do?
Thanks very much,


[ May 22, 2003, 12:27 PM: Message edited by: JHZR2 ]
About the only thing you can do is try it and see how it does. Look for things like consumption and any unusual engine noises. It will probably do fine.
I wouldn't worry about the low number so much, Mobil 1 0w40 has a viscosity at 40c which is higher than most 10w oils, at 83.5 at 40c I believe. Most 10w30s out there are closer to 60cst at 40c.
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