Oil - "W" DOESN'T Stand For WEIGHT! In fact, weight is not actually a thing

Lake Speed Jr. puts it better than I can. Weight is not a thing for oil and It’s surprising how many professional engine builders don’t understand this… But then again, it’s the on track results that count…


I knew about the Winter part from my early days turning wrenches, but did not know that the xxW was measuring torque resistance and not flow rate. Science is so cool like that.

The only time an oil number means anything directly is when an oil is reported as an ISO number. When it is the ISO roughly equals the viscosity in centistokes at 40C. SAE standard viscosity grades are crazy wide ranges. 20 grade SAE can be anywhere from 35-70 cst viscosity.

The W does indeed mean winter. And the method for specifying the W (or cold cranking) grade for SAE motor oils is not straightforward or consistent. Each W grade has a different temperature that it is reported at in accordance with SAE guidelines.

Thankfully the applications I used to deal with didn’t use any archaic AGMA of convoluted SAE specs and instead all followed ISO. Much much easier to work with!

The chart below crosses standard SAE grades to other specifications.


Learn something new everyday, I had no idea.

I suspect that in an emergency, with no other 2-stroke oil available, 75W140 GL-5 gear oil would work really well. It would also stink like a den of skinned skunks, but you can’t have it all.