Matthew Currie said:
Others may differ on this, but I prefer thin oil as long as pressure is good. The theory here is that as long as the oil is cushioning the bearings, it's doing its job, and the thinner it is, the faster it flows with less effort, and the more heat it carries away. If you have enough pressure, then you have enough pressure. The engine really doesn't benefit from more than enough.
I agree,
BUT I wouldn't use an oil thinner than recommended for the conditions. The additional logic behind that is, viscousity "usually" is related to shear or film strength of the oil. Thats the resistance of the oil of being force out from inbetween two metal parts and allowing metal to metal contact.
So even if the thinner oil gives you adequate oil pressure, it may not give you enough shear resistance to prevent metal to metal contact. Things like some of the valvetrain don't rely on oil being pumped into a bearing, but rather oil being dripped or sprayed in between rocker tips and valve stems. So even if there is enough oil pressure, if the film/shear strength isn't high enough then you may get metal to metal contact.
As well, I think the manufacturers probably have a big debate amoungst their staff about recommended oil viscousity, with the pressure leading to recommended the thinnest oil that is safest to use, so that they can get higher mileage and energy savings on the vehicle. I'm sure there are engineers that scream that the viscousity should be higher, that under some conditions that low of a viscousity could lead to metal to metal contact and extra wear.
THUS, I wouldn't use less than recommended viscousity, ACCORDING TO CONDITIONS. Look in you drivers manual, there are temperature ranges that go along with the viscousity recommendations, pay attention to those, for some of use a different viscousity than all the others use may be in order.
I said film/shear strength is usually related to viscousity, thats a general trend type deal. All things being equal, higher viscousity oil inditical to the lower viscousity in all things except viscousity, the higher one will have more film/shear strength. Qaulity, composition, additive packages, conventional/synthetic all affect that as well.
Good Quality 5W-30 Synthetic may have more film/shear strength than poor quality 20W-50 conventional oil.