I've put Rotella 15W/40 in the motor. I ran this rebuilt motor for a few hundred miles on Castrol 20W/50 but because of all that has been written about changes in the current oils and cam problems I've decided to go with the Rotella. I'm wondering if it would be beneficial to add a liter of STP? What ever happened to "Molyslip E"? I havent seen any lately at the Auto stores. Karl