A buddy gave me 3 or 4 quarts of mobil 15w50 synthetic that he had for a while. It says tri-syn on the bottles, I would say it's maybe 5 years old. But they're unopened. Think I could mix this with new mobil-1 5w30, just to get some use out of it? I don't have anything I really want to run 15w50 in, other than the 351w in my boat.