I would make sure that you talk to you builders before exp. with a $6,000 motor. I paper work from Ken Joyce Racing that says to put 20 / 50 in, I know 600 says 20 / 50, Hank Scott has told other drivers I race with to use 20/50, and my builder Reed racing Engines said 25 / 50 penzoil. So, do what you will but I would talk to you builder and see what they have to say.
My motor is from a guy that over 4 years and a track champ at LakeErie speedway, very fast, has had only a head stud come out in all that time and he ran thicker oils. I broke a crank last year too, but it was the driver. Stood on the brakes while the motor was at 10 grand and did not hit the clutch. That was 1250, which in the past had problems with the cranks braking..