Originally Posted By: CATERHAM
Choosing a suitable oil viscosity for any engine is not difficult and it certainly isn't rocket science.
There is a minimum optimum and maximum optimum operational viscosity for every engine. A 5W-30 engine oil will place you in the middle somewhere with any engine at normal operating temp's (80-100C). If the engine is worn or operates at higher oil temp's then a heavier oil grade may be required. If the engine spends a lot of it's time at low operating temp's then a high viscosity index 20wt oil may be more appropriate or an even lighter grade (Toyota and Honda will be coming out with 0W-16 grades in 2014).
So what is the minimum optimal viscosity for an engine? Well an oil that is just heavy enough to meet the oil pressure test spec's provided my the OEM is a good place to start. The maximum optimum viscosity would be an oil that is light enough so that the oil pump is not in by-pass mode at the maximum oil flow rate (that would be at elevated engine rev's) when the oil is up to temperature.
I told it wasn't rocket science.
Caterham,
While I am not against thin oils by any means (there, I said it just to clear the air before I start!!!), I would like to point out a few of my own observations / thoughts.
I can agree that almost any passenger car on the road today can run on anything from 0W20 to 20W50 and live a reasonably long life. I can also agree that it is not smart to put 20W50 in a car driving across the street everyday, and that 0W20 on a race track may not be appropriate either. So, as you state, just meet the minimum oil pressure spec as listed in the service manual, right???
However, I see a few possible concerns with this. Firstly, this is not a "spec" it is a MINIMUM spec. I will use my old 2005 Nissan Altima (3.5 L V6 engine) as an example, since I have a lot of data and experience with it. The service manual clearly states that oil pressure must be HIGHER than these numbers. It says nothing about any maximum. It states rather clearly that when an engine cannot meet these minimum numbers with specified oil at full operating temperature (oil temp), then it needs an overhaul.
Using the same car with some numbers now... It states that the MIN oil pressure at full hot idle is 15 psi, and it gives a second test spec of MIN 45 psi at 2500 RPM. This obviously assumes that all healthy engines exceed these numbers on spec'd oil. With a MIN spec of 45 psi at 2500 RPM, how on earth would you not cause the oil pump to bypass at 7500 RPM??? There is no way that this could be true unless you use a 0W10 racing oil, in which case the minimum specs would not be met at the two test RPM's.
Using another vehicle, my tow vehicle (Chev Venture) has a relatively low bypass setting, and will still cause the bypass to open up at hot idle anytime above 4500 RPM. This is with a 3.1 cP HTHS synthetic oil, which is inline with what GM specs. This is also after long highway runs with my trailer, often engaging and remaining in 3rd gear to climb long uphill passes. 270,000 km so far and runs well.
My friend's Mercedes S500 with the spec'd 0W40 always went into bypass about 2500 RPM prior to hitting redline even after long highway runs, and this vehicle is equipped with a very large oil to water heat exchanger which maintains the oil temp at 90C once warmed up (it will make its way to 100+ C on the highway but returns to 90C with a bit of idling). This car has about 340,000 km on it at this point btw.
Are you suggesting that all of these vehicles that routinely enter oil pump bypass in daily use are not using the correct lubricant? Do you honestly think that the engineers that designed these engines don't know that they will enter pump bypass during normal operation? Ultimately, do you think that they did not already take that into account and ensure that the engine, despite being in bypass, still has ample flow of oil (i.e. more than the min required for proper lubrication, even though bypass is active)?
I can't say with any certaintly, but then again, neither can anyone else unless they happen to work on the engine development teams of a major manufacturer.
