Originally Posted By: rjacket
Someone on these forums once told me that oil pressure would be constant due to the oil pump.
I'd find that dubious at best. If that were true, we could all run whatever the heck viscosity we wanted.
Oil pressure changes with RPMs. Pressure will also change when viscosity changes, regardless of whether that change in viscosity is due to different temperatures or due to a different grade oil. I would agree that sometimes the pressure doesn't "seem" to change. That's particularly true with cars that have idiot gauges and those gauges that don't have very high precision scales or a lot of accuracy. It was hard to compare oil pressures in the old Audi since, while the gauge behaved as expected (up and down swings quite apparent with revs, expected behaviour at cold starts versus hot idle), it was very difficult to closely compare pressures under different conditions since the thing was calibrated in bar (which is a large unit) and there were very few graduations printed on the face.
Heck, even anecdotally with my old F-150, when oil temps got hot and the thing was at idle with a 30 weight oil, the oil light would flicker, prior to the rebuild. With fresh, non-fuel diluted oil or 15w-40, it would never happen, no matter how hot the oil got.
Originally Posted By: rjacket
Based on that input, I was thinking all you needed to know was the oil viscosity at operating temp as per the product sheets, and the parameters that the manufacturer used when specifying recommended oil weight.
What else on oil pressure helps make decisions beyond those two points of reference?
I'm not so sure on that. Some vehicle literature I've seen lists pretty wide oil pressure ranges; coincidentally (or not), those vehicles also have fairly liberal oil recommendations.
Someone like CATERHAM is much more versed on kinematic viscosity and its usefulness in this situation. Haas and CATERHAM are well versed in using pressure and temperature to choose a suitable oil. From my perspective, you need the pressure values expected at operating temperature at given RPMs, from a service manual or similar source. Occasionally, they'll give a range as to what's acceptable at operating temperatures at idle and one or two other tach points. If you're outside of those values, there's a pretty good chance you're running too thick or too thin.
We also have to remember that manufacturers' specifications for pressure (and for specifying a recommended oil weight, as per your mention) depend on the oil being at a certain operating temperature or range, without completely ignoring cold starts and short trips, either (otherwise we'd all be using straight weights). For my hypothetical one block trips in my old F-150, even a 0w-20 would be too thick.
My view is that the engineers have to compromise. We know that say, a Ford Focus, isn't going to blow up if it's using 15w-40 for constant highway use (cold starts are another matter), and Dr. Haas isn't going to have catastrophic failure using 20 weight oils to get groceries in a supercar.
Simply put, lots of lubes will work. Some are just more optimal than others for different circumstances. My late model G37 specifies 5w-30. It spends almost all its parked time in a heated garage. 10w-30 certainly wouldn't be an absurd choice. My 1984 F-150 specifies everything from 5w-30 to 20w-50, depending on temperatures. It spends all its time outside, including -40 C. A 0w-30 at -40 is certainly thicker than 10w-30 at the 10 C in my garage. Basically, I'd feel a lot more comfortable running 10w-30 in the winter in my late model G37 than I would in my old F-150, even though it's speced for the F-150. Engineers have to make their recommendations based upon their best information as to how a vehicle is going to be used and what oils are available at the time and in the target market.
Originally Posted By: rjacket
Lastly, how many cars have this limp mode issue?
That's a very good question. Hopefully, the number isn't too high. The solution to high oil temperatures is to improve oil cooling, not put the engine in limp mode.
