At a 140,000 miles. I think that engine looks OK. Ive seen alot worse and not too many better with that high of milelage it looks like a motor with about 90-100,000 miles and its got 40,000 more than that. I also dont think its fair to bash Castrol or RX since we dont know how bad or Good it was before at 100,000 miles. I would just run it and change oil as you have been. As far as Synthetics, I think conventional oil has came a long way and the comparison is marginal at best. This at reasonable change intervals of course.