I have a '90 Cherokee with the 4.0 liter inline 6 engine. It's been a bit of a project car for me over the last year or so, and it's just now getting to be a daily driver. It's got 98,000 miles on it, and the previous owner (my dad) rarely changed the oil...and allways used the cheapest oil he could find if he didn't take it to jiffy lube. The first oil change I did on it, I don't know how long it had been, but the oil was DISGUSTING! I could have used it to re-pave my driveway. (the last few years of it's life he cared even less for it than when it was new, so it had probably been in there 3+ years) Now, considering that, the engine still runs pretty good. I changed the oil, with 10w30 dino oil and a can of marvel mystery oil, and drove it about 200 miles before I noticed the drain plug was leaking. So, I put some Seafoam in the oil, drove it about 20 miles, and changed the oil and filter again...and the oil was pretty dirty. Now I've driven it about 100 miles since then (second oil change in 300 miles) and the oil on the dip stick is looking pretty dirty already. What gives?? Is my engine just THAT dirty on the inside? Or are there some other problems I should look for?