[QUOTE=Fubar569]I have no idea whether BMW's recommended oil change interval will be detrimental. The BMWs are leased and go back to the dealer to be resold as pre-certified (pre-abused in my book). I can tell you that the oil on the dipstick at about 29K looked like black honey. Even after it was changed at 30K, it looked liked it needed to be changed, in fact it was hard to tell that it had ever been changed. IMO, I would never go more than 7500 miles on Mobil 1 in a Vette (my 95 gets driven less than 3000 miles a year so I change it at least once a year, and I always change my Z when the oil life indicator hits 25% which comes to about 4000 miles or about once a year also), especially if you are jumping on it alot and using an air filter other than paper.

 

I use 0/30wt in the vette and 0/40 in the benz. I don't know why Mercedes spec's 0/40, rather than 0/30.

 

I thought this was a tremendously informative site about engine oils, directly contrasts much of what has been said in this thread alone - might be helpful - I think I originally got it from the forum here to begin with!

http://63.240.161.99/motoroil/index.html

 

No worries with that. I've used the 10W30 M1 when the 5W30 wasn't available.

 

I've read this guy's articles and while I do agree with much of what he says, I believe that he takes some good information and stretches it. He states:

Remember that most engine wear occurs at startup when the oil is too thick to lubricate properly. It cannot flow and therefore cannot lubricate. Most of the thick oil at startup actually goes through the bypass valve back to the engine oil sump and not into your engine oil ways.

Many vehicles and oil filters do not have bypass valves. To assume that all do is not accurate. To state that the oil absolutely cannot flow at startup is simply exaggeration. He says that if it cannot flow it cannot lubricate. So what about sealed bearings? What about teflon tape? What about grease? Doesn't it provide a layer of lubrication so that parts move against each other without metal to metal wear? Would it always be better if it were thinner?

he also states:
I remind you that a 10 or 5 or 2 weight oil is still too thick to provide lubrication at startup. They are all too thick at startup. There is currently no engine oil thin enough to operate correctly at startup. They all cause excessive wear at startup. Again, we are discussing the needs of my single hypothetical engine for around town driving.
Hyperbole and exaggeration.

The author addresses all automotive oil lubrication as if only hydrodynamic lubrication matters. There are also boundary layer lubrication issues to consider. See an actual research paper available http://www.iantaylor.org.uk/papers/IMechEFE2000.pdf

Much of what this paper shows actually agrees with AE Haas in general, as do I. However, it also shows that based upon some good information, Haas is making some statements that are not so good. I believe that reveals some misunderstanding of lubrication theory on his part. In the IMech web article look at the chart - Table 2. Notice that while other components incurr a power loss with the use of thicker oils, the Valve Train power loss decreases with the thicker oils. The lubrication is better. This is an aspect of metal-to-metal lubrication that defies some of what AE Haas says. Why? Because different load/pressure/flow circumstances exist between piston to cylinder wall lubrication and valve train lubrication.

Here is a simple example: Take a steel plate set at an angle of 30* and cover it in a very, very thin lubricant like WD-40. Would you be able to walk up the plate. With difficulty most likely. Now put a thick layer of 90w gear oil or 50w motor oil on the same plate. Much harder to traverse. Same surfaces, same pressure applied by your shoes on the plate, different results. The internal viscosity of the gear lube will keep it in place so it doesn't all run off. That keeps more lubricant film available to make you slip as you try to go up the plate. In some parts of the engine this can be important.

The IMech article shows that current testing and software modeling agrees with at least one aspect of what has been posted in this thread. The difference between 5w-30 and 10w-30 in the modern automotive engine is negligible. Using the 5w-30 is slightly better than the 10w-30 with respect to fuel economy, but the difference between the two is not enough to warrant making the trip back to the store to swap.

For some real world calculations of lubrication film thickness based upon all the parameters that can have an effect, see the Timken web page at: http://www.timken.com/lubrication/engineering.asp