I recently purchased a 91 Stealth R/T TT and it is now time to change the oil. Oil capacity is 4.5 quarts, I was planning on 1 quart mobil 1 15w-50, and 3.5 quarts of 10w-30. The reason for the 15w-50 is to thicken up the 10w-30 which I believe is just a little to thin for a high horsepower turbocharged car. I also noticed mobil 1 5w-40 which I had not heard of until yesterday. Should I stick with my orginal plan of mixing 15w-50 & 10w-30, or try 5w-40?