Major sponsorship of a Nascar team can run in the area of $15 million per year. That's a pretty big chunk of change.
AMSOIL is a small company in a niche market. Years ago, they did sponsor one Indy car at the Indy 500. Think he made 3 laps before going into the wall. That was the only car with AMSOIL logos on it, but a number of cars were running AMSOIL gear lube.
AMSOIL is into some race sponsorship now--limited, because they feel they get more bang for the buck being a big fish in a small pond. So, they are big in snowmobiling, motocross, some boat racing. And the 2-cycle oil sales have increased dramatically.
There are a pretty fair number of Nascar teams running some AMSOIL products. They buy them!
They don't run AMSOIL logos, because AMSOIL doesn't pay them to do so. Sure, Al and some of the others know which teams are using the products, but are not going public with the info--why take a chance on antagonizing a user (who may have a sponsorship from someone else)?