Expert: Ben Deutschman - 4/12/2011

Question
Hello Ben,

I own a 1993 Jeep Cherokee Sport-6 cylinder, automatic, A.C.- 4.0 litre, 4WD/2WD-248,000 miles. Got it secondhand 6 years ago from older couple at 209,000 miles, well taken care of. Always had small oil leak from valve cover gasket or oil pan-about quart every 2 weeks. Not coming from rear seal or oil pressure switch-that's new.

Have always used Castrol High Mileage 10W40 until recent oil change. Switched to Valvoline Synthetic 10W40. Engine runs smoother, pick-up of around 3 p.s.i. on idle, and oil leak has practically stopped. Haven't added any in over a month. The oil looks dirtier on the stick than when I was running the Castrol High Mileage. It appears quite a bit darker and appears to have tiny particles of dirt in it. Someone told me the synthetic oil was able to get in and clean parts the Castrol couldn't, but that still doesn't explain why the oil doesn't leak anymore.

Can you help explain any of this?

Thank you very much.

Mike E.

Answer
I've never used synthetic Motor Oil, so I really can't say why you'd see more dirt in suspension in the oil. From what I've heard though, synthetics can find their way past the smallest opening, and leak out, which seems to run completely counter to your experience.