oil grade

[mdoyle]

You guys can't compare various motors and grades of oil and draw any conclusions about performance. The viscosity oil is specified for a particular motor design and it's not necessarily about quality this or quality that.

The main key is this- bearing clearances and their L10 life are based upon the formation of a hydrodynamic wedge of oil between the running surfaces. The viscosity is specified for that clearance and a few other variables, but mainly that one clearance.

What happens with a sleeve bearing like the ones on the crankshaft, connecting rods, and camshaft is this: when a shaft turns inside a cylindrical bearing that is oil lubricated, instead of the oil flowing outward like you'd expect it pushes inward from both sides forming a pressure ridge in the middle. This wedge formation supports the shaft that's turning inside it.

Since the wedge isn't present at start-up, that is when most of your wear occurs. It's also the reason turbo equipped cars usually have a pressure reservoir to supply the turbo bearings after shut-down while the impeller is still spooling down.

Stick to the manufacturer's oil specs, avoid "snake oils" that make absurd claims, and use the right SAE ratings for your car and you'll do well.