I use 15W50 but also try to make sure the engine has some oil temperature in it before making passes. Thinner oils are better if you don't spend any time warming it up at the track for sure. The thicker and/or straight weight oils don't flow as well cold. In some other applications we've failed engines with heavier weights at cold ambient temps because while it has pressure, the oil isn't flowing to all of the appropriate bearing surfaces.
Yep, even after a brief warmup down the return road, the car will usually gain quite a bit due to all the oils and lube being warmed up after a pass.
