egroups.com:

30 is fine though towards 40 is better,
but if you have less than that at anything above about 1500 revs and
you're not running sewing machine oil, you don't have enough to keep
from scraping bits of metal together on your cam or your bottom end.

It isn't the oil pressure that keeps metal bits apart, it's
the oil itself.


OK, fair enough for the Valve train, but the bottom end?

Oil pressure is needed only to get the oil to the
places it has to go. If it's a light oil, less pressure will work, and
a heavier oil might need more pressure to keep it moving. My A-65
seldom sees more than 25 or 30 psi in cruise, and works just fine. If
the weather is really cold I'll use an automotive 15W40 oil in it to
keep it happy. Beware, though, of using an auto oil if the engine has
hundreds of hours of aero engine oil operation; the auto oil will
loosen a lot of accumulated crud in the case and that stuff will get
pumped all through the system or maybe plug up the screen.


You've not been using AD aviation oil? That'll do the same thing if you've
been using straight mineral.


Bertie