I have an air cooled R1100S with the factory oil cooler. The bike has 3000 miles and I wanted to switch to M1 15W50 but the daeler said not to. He said just to stick with the BMW brand 20W50 dino. He claims that its very good oil and has the proper additive package for the air cooled boxer engine. I was wondering if this is good advice? My question is, since an air cooled engine would be harder on oil, why wouldn't I want to use a fully synthetic oil?