thankyou Vspecialty&Tom for the explanation, just when i thought i knew alot about oil apps you took me back to school. i read your post in the gen3 section and now understand why the 40 switch was made for gen3's considering the different design of the engine related to oil flow/pressure/temp:
http://forums.viperclub.org/srt10-srt10-coupe-discussions/610238-gen-3-oil-change-tips-5.html
so regarding the Gen2's , we're talking about using 0/40: 0 for the startup efficiency, and 40 for the added protection with slightly less power. but what about how it affects flow/temp/pressure? How would it affect each of these?
Is added protection needed when many have reported clean gen2 oil samples (from analysis) of stock Mobile1/30 at 5000+ miles?
Why not the best of both worlds, mobile1 0w30?
Why does our manual and DC say only 10w30 and not 5w30 OR 10w30 like most other cars?
Sorry for all the questions, I just don’t want someone going buy the logic; Gen3’s run it ,Why can we! Without considering the different designs of the engines.
At low temperatures, an SAE 0W will flow more easily than an SAE 10W.
At higher temperatures, the SAE 0W-40 thins out less than the SAE 10W-30. Yes, all oils are formulated by starting with the "thin" end and using a "viscosity index improver" additive. The additive makes it behave like a thicker oil than it really is. But get your head around this - at the higher temperature that this characteristic is measured (100C) the oil is very runny, vs. slow and thick when really cold. To repeat, the 0W-40 gets thinner slower than the 10W-30.
Oil pressure is a measure of resistance. If you could put your thumb over the outlet of the pump, the pressure would be very high, but with no flow. So I would like to modify Dan's explanation a little and suggest that the 10W-30 would be thinner and leak out the side of bearings thereby not delivering enough lubricant to the next bearing and the next one after that. So fundamentally the volumetric flow rate is the same out of the pump (since it is a constant displacement pump) but more oil is actually delivered to the bearings because less is lost through leakage on the way there. Since oil pressure is measured at the outlet of the pump (I think, I'm a throw-back Gen 1 guy) it will read higher, but that doesn't tell you how much oil got the the last bearing. Because the pressure is higher (or at the relief limit) the oil is under more shear and theoretically be hotter. But I doubt you'll see anything.
Is it needed? If oil analysis says it's fine, then no. You can't argue with success. However, tracking conditions may change the used oil results. And if the quart of 10W30 costs the same as the 0W40, you get extra insurance and no debits with the 0W40.
You could use a 0W-30, too, except for Dan's description of the Gen 3 engine. But you are thinking on the right track.
The Gen 1 engine was introduced in 1992, so designed in 1990? The Gen 2 engine was introduced in 1996, so designed in 1994? You have to look at what oil was available in those years. And having had a little contact with the OEMs back in my oil company days, I know the OEMs have one large "house" oil tank and little desire for the care and permitting process to get another tank for a lesser used oil. Once an engine is qualified with a certain package, all the warranty issues are based how that package performs. So it's convenience and defending against field problems.
A well designed oil system makes a big difference. The typical NASCAR V8 has 930 HP, runs 9600 RPM or so and can easily run 0W-30 or less. The restrictor plate engines were 420 and they couldn't find an oil thin enough (yes, they were looking for 5 HP then.) Cosworth XFE CART/CCWS engines were 850 HP 14,000 engines and used 0W20 all day long. The design target is to deliver the oil, not necessarily pressurize it. If you deliver the oil to the unloaded side of a crank bearing, the rotating crank will pull the under under it and into the loaded side. At the minimum oil film thickness the pressure is thousands of PSI - the oil pump is not going to push oil in against that. The oil pump is simply a way to deliver the oil. The rule of thumb of 10PSI per 1000 RPM came from what Dan was referring to - that at some points inside the crank the oil has to travel against centrifugal force to get to the next crank throw. If the pump couldn't overcome this, the next crank throw ran dry, which frequently happens at high RPM at the back end of the crank. But the pressure on your gauge really has nothing to do with how well the moving parts are lubricate.
