I've studied this topic extensively in my job as a marketing and branding consultant and as a FC driver in the licensing process.
To add to Alan's post, sponsorship activation and measurement continues to evolve. A fact about NHRA fans according to NHRA and Scarborough Research: NHRA fans are fiercely loyal. More than any other major sport, drag racing fans are more likely to buy a sponsor's product over a non-sponsor's product and more likely to switch brand loyalty if a brand sponsors the NHRA. This type of loyalty and active fan participation is hard to find.
While it is an effective start for measuring the value of a sponsorship, most companies have evolved beyond the measurement of logo placement on television. Companies create activation programs designed to meet specific business objectives. This could be a lot of different things:
1. Building awareness (not a problem for Castrol, but MavTV might have this as one of their objectives; in this case signage on the car is important)
2. Product trial (for example: Traxxas giving fans the opportunity to play with their RC cars at the events)
3. Employee performance incentives (in-event hospitality is great for this)
4. Building customer loyalty (hospitality, on-car branding, in-store promotions, driver appearances, etc. can be used to this end)
Basically, sponsors have a lot of varied business objectives, and proper activation should take the form of helping Joe Drag Racing Fan enjoy the race experience even more than if that sponsor was not involved. If a sponsor is getting in the way of your experience then they're doing it wrong.
As racers, it's our job to build a valuable a pool of assets to help the sponsor achieve their objectives. Force is great at this, with his roadshow, all of the appearances they do for sponsors, the old reality show, events at their shops, website and social media involvement, merchandising, etc.
You can see by a small sampling of possible sponsorship objectives that measurement gets a little complicated, but marketers find ways. And the measurement dilemma isn't restricted to just sponsorship, many marketing activities are difficult to quantify.
Bottom line as Alan put it, if a sponsor stays, they're getting their money's worth.