Hey guys and gals, I recently bought a used 2008 Kawasaki Brute Force 650 with only 3800 miles on it. I noticed that after about 15 miles of use that the oil has dropped from the full mark to about half way down the hash marks. This has happened twice. The engine does not knock, runs great, no power loss, no leaks, and does not smoke at all even during start up. What could this be? My first thought, maybe the previous owner put a lighter cheaper oil in? Synthetic? Thanks In advance!