when oil is the most important part of your engine. just bite the bullet and change it. Petro-Can DURON is Semi-synthetic. they have some weights that are full synthetic. and I think 15W40 is full synthetic, im not sure. But I just run the Amsoil 15W40 and man does my truck love it. I get it for free because my work buys 205L drums of it and they just let me take it when I need it haha.