Years back I was able to take a few classes for an insurance license I was after. I was lucky enough to be setting in class with an engineer working for Mobil Oil. Mobil 1 had not been released to the market yet. Being young, and into Corvettes, and this engineer also driving a vette, well, we became friends right off. After a few weeks oil became a topic of discussion, and he was testing Mobil 1 in his new vette. He told me he was changing oil every 10K miles. I was astonished because in those times, it was 3K... So, in those few weeks I was introduced to synthetics and started having my oil analysed by a lab he suggested, and I was allowed to participate on those early years. WE ran our big trucks for 10k oil intervals, so I knew it was possible. Long story short, I was spending the big bucks for Mobil 1 after it's release, but was changing my oil every 7k, and have continued this 35 yrs later plus or minus...lol That doesn't mean other synthetics aren't good, just what I like to run having this history. I hear good things about Amsoil, and did try some in the Harley. I didn't see any difference other than price... What I wouldn't recommend having seen a lot of engines torn down, is pennzoil, Kendall, quaker state. You had better be changing those every 3k, because if you don't you see a lot of build up on the valve covers. They don't like heat, where synthetics do a better job. Just my opinion.