does it have a working oil sender / lamp.. or mechanical gauge?
if you think it's cheap oil.. or thinned? why havn't you changed it yet... that lil 3 cyl don't hold $enough$ oil to even play around guessing about oil quality.
go put a double dose of seafoam in the fuel tank, dunp a quart of atf into the crank case.. start and run her till warm, kill engine.. drain oil.. repalce with some good 15-40 and don't forget the filter.
then run it.. pay attention tot oil indicator and exhaust color for a bit.
ps.. I'm also a fan of the lil blue bottles of diesel fuel cleaner from prestone.. found at palces like walmart. I got a load of watered fuel once and was making my powerstroke run like junk.. one blue bottle and some seafoam fixed it right up.
check yer fuel fitler too.. IE.. CHANGE them..
soundguy