I picked up a good deal on a case of Formula Shell 5w30 SN rated conventional. Oil cap on both cars (Kia & Hyundai) states 5w20 but OM says 5w30 is acceptable . I could either run the Formula Shell by itself OR I could mix with a stash of Pennzoil Platinum Synthetic 5w20 I have on hand . Both are SOPUS products (if thast means anything in a potential mix). Which would you choose ? Conditions are short to moderate suburban driving with some high way - both vehicles are still under warranty .