Originally Posted By: gfh77665
Ssshhhh! Don't tell anyone, but I am running (almost free) straight 40wt. PYB in my Chevy truck this summer. MPG's, starting ease, oil pressure, RPM's, everything still the same. Remember...Quiet please!
I have Spock ears... I heard ( : < )
The part in red is impossible. A 40 grade is thicker at all temps than 5W30 or 5W20, so oil pressure will be higher at any given oil temp. It's worse with a straight grade oil. If you aren't seeing it on your gauge, it's an OE dampened gauge or one of those "idiot-lights-with-a-needle" so that it doesn't show the difference. My statement is based on physics but I have verified it with accurate gauges on two trucks. With one (diesel), when I went from 15W40 to 10W30 HDEO, there was a 4-5 psi drop in oil pressure at the same oil temp. In another (gas) truck, going from 5W20 to 10W30 there was a 5-8 psi increase.
I'm not saying that 5 psi one way or another is hugely significant on a warmed up engine, just that the difference is there whether your gauge is indicating it or not.
In Texas, you can get away with a straight grade oil more than most.
For the benefit of others, one of the disadvantages of running a heavier-than-spec'ed oil is that you will put your oil filter closer to bypass with the increased pressure of a cold start. That means you really need to feather-foot while the engine is cold because if you don't you will be blowing unfiltered oil past the bypass until it worms up and thins out. This is another thing I have verified with one of my trucks. I know the bypass cracking spec for the oil filter I use and I have a differential pressure gauge set on my engine so I can read the pressure on both sides of the filter.
I am currently running 10W30 in an engine designed for 5W20... an experiment. The cracking DP for the bypass valve in the filter I use is 8psi and I bump up against that even on a 70F start when the engine flares upon start (EFI). I have to drive really easy until the oil reaches about 150F (yes, I can also read oil temp) before I can engage in spirited driving. Even with the oil at 190F+, when I bump up against the rev limiter at 5500 rpm, I am bumping up against the bypass DP as well. I have just finished datalogging with the 10W30 and will be changing the oil back to 5W20 and doing it all over again to see wassup. Will also be testing the new DP of some popular oil filters just for grins.
And there is another price to pay for thicker-than-recommended oil and for single grade oil that doesn't flow well cold. "Cold" is relative. To the oil industry, anything under 100F is "cold" and if you look at the viscometrics of a straight 40 grade at, say, 70F, it's running at 486 cSt (40c scale, about 32 cSt on the 100c scale) which puts it thicker than 60 grade motor oil and at the upper end of 140 grade gear oil. That's at a relatively warm 70F start. I shudder to think of a 40 F start. A multigrade oil does much better.
This was a borderline hijack but I thought it was relevant to the discussion.