I concur with Steve. I am running Royal Purple
synthetic racing
oil and I believe it is 15 W 50. Labeled as Racing 21.
Just a word of warning from a couple of the engine builders around here:
Do not use the 0 W 15 weights if you are doing any track time. The racers that run these lower weight oils can afford to rebuild their engines a lot more often than most of us. Plus the horse power gains are minimal. I have no personal opinion, just what I have been told by several racers and machinists.
Clois