Oil Weights and Numbers

Seeing the thread about a DE who seemed to have the wrong idea about
communication frequency areas on a chart made me think of something
that's been bugging me since my check ride.

The DE asked, "What kind of oil do you put in the airplane we're going
to fly?" I told him the plane used 50 weight ashless dispersant oil.
He replied that the plane actually used 100 weight oil. I was pretty
sure he was wrong but I let it go.

Later, I grabbed a pint out of the box in the office and it said "50W"
but also had a "100" above that. I see why he made the mistake. Or did
he? I dug around on the AeroShell web site and found products labeled
similary:

OIL W 100 SAE 50 - ASHLESS DISPERSANT

Looking through their FAQ, it appears that this a multi-viscosity oil.
Is that right? 50-100 multi-weight? Why is the label different from
automotive oil (e.g, "5W-30")?

What's really confusing is the W in front of the 100, followed by SAE.
Maybe there's an organization other than the SAE that sets the standards
for aviation oil?

-Scott