Good point Motorbike that Nanney's "engine appears in good condition still w/o need for an oil pressure guage and no real need to change brands or viscosity." On a similar note you said my analysis showed that I could run 10w30 Maxlife and to think about what my analysis taught me. Well maybe I need Dyson, but here is my understanding:
Running a 5w30 (Durablend) when I put in the oil pressure gauge I noticed pressure was running 36 psi hot at 2000 rpm and 16 at idle. When I ran it hard the hot 2000 pressure would drop to 30. Spec for this engine is 40-60 at 2000, so I drained two qts and added two qts 20w50 for approximately a 10w40 (or maybe a thick 10w30) and pressure went up to around 44 psi. While the 30 weight was OK by the tough and dirty rule of 10 psi per 1000 rpm, I felt better having it within spec and the 10w40 raised the idle pressure 6 psi higher too.
Now, the puzzle is why is the pressure on the low side with xw30? The UOA seems to say the engine is in great shape without excess clearances and the consumption is minor (possibly more related to heavy foot than engine condition), so why the low pressure? The only thing I can figure is what my neighbor (an engineer at Ford) told me: that it could be a weakened spring on the oil pump relief valve. So at this point, given the UOA, the weak spring theory seems more likely.
Anyway, I figure I am in the spec range (got it down to 40 psi running 4th gear at 80 mph into a fierce head wind for about 10 miles one winter day), so xw40 weight probably not a problem and I'm figuing so long as it is summer I might as well run 15w40, since it will have less viscosity index improvers and thus would be a bit more robust for summer temps.
Also, the 10w40 has a higher HTHS than the 10w30. The 15w40 should have even higher HTHS and that should be a good thing. And on that note, why not use the pressure gauge to select a higher viscosity grade that still falls within spec? Higher viscosity is not necessarily bad, right? I've only had the pressure gauge since last Sept and so I might be suprised at what happens when it gets consistently in the 90s (F).
[ April 18, 2004, 12:12 AM: Message edited by: TallPaul ]