"motorcycle" oils sure do that sometimes. Obviously you can use a range of oils based on where and how you ride. Diesel oils work really well and most times are not overly expensive. Standard rotella t 15-40 is about 14 a gallon or so, maybe less, at walmart. I just bought my rotella t6 5-40 synthetic at walmart for 21 a gallon.
Whatever oil you do buy, I would make it a synthetic. It just holds up better than a conventional oil does.
A couple posts up I put a link to another site. It is a long read but might help clear things up for you.