Haven't had to change the oil yet in my SC, but in my last car (240sx) I used the seafoam in the crankcase as well as the intake. I was told to do the crankcase first, run the engine for 5-10 minutes, then do the intake. That was the first oil change I did in that car, and I switched from conventional to synthetic - never had any problems. But I would definitely suggest changing the oil immediately if you use it in the crankcase.