My father attended a Mobil lubrication seminar where you get a free case of Mobil 1. Well, some people didn't show up and the Mobil reps didnt want to take it back with them. They only had 5w30. So now we have 10 cases of Mobil 1 5w30 SuperSyn. I will probablly just use it as is in his 1990 Pathfinder with 140k miles and no leaks. It has been on Mobil 1 for awhile. The other two cars are a 2001 Grand Prix GT and a 1999 Grand Prix GTP. I just don't feel comfortable using such a thin oil in them... especially the GTP. The GT is on Mobil 1 10w30 and has a ton of engine noise. The GTP is on a diet of one gallon of Delvac 1 5w40 and one quart of M1 10w30 and is nice and quiet. If I added one quart of 15w50 Mobil 1 to the four quarts of M1 5w30... would it make it any thicker or would I just be wasting time? Thanks guys!