re: Do you use synthetic oil?

There is, 2nd exit South, one block off interstate. Use it for both vehicles. Use synthetic on both.

Very few people get 10,000 miles on an oil change and higher mileage vehicles may get less than 3,000. Check your oil level, oil color and smell every 1000 miles. If it's low, add oil. If it's dark or smells burnt, change it. Going 10,000 miles without ever checking your oil, just because a manufacturer says you can, is a good way to get yourself a new engine.

Your engine doesn't give a crap about whether you switch between conventional or synthetic. Current API/Euro/Jap oil specifications require way more detergency than what was required even a decade ago, your engine will look just as clean inside on regular oil with proper service intervals as it would with synthetic. I used M1 in my truck for the last few years due to severe service and winter temps, now that I'm back in the South I'll be going back to Motorcraft oil with the same mileage OCI that I did with M1.

Replying to original post:

Regular oil in my Ford Expedition. 250,000 miles on it now. Change it regularly every 5, 5000 or so miles.

Synthetic on my Harley. ONLY synthetic.