in the summer I usually use mobile 1 super synthetic 15w-50. I was considering using royal purple because my friend gets it at cost the two grades of oil they carry that are the closest to 15w-50 are, 15w-40 and 20w-50 which one do you think i shoud go with.

 

i use 5w-30 mobil 1

 

in the summer i'd run the heavier, in winter i'd switch to something like 5w-30 or 10w-30.