Anyone have positive experience with using quality synthetic oils in high mileage vehicles? In particular dealing with engine flushes for extended oil drains (15K and higher). I have been switching cars with over 90k using motor flush. Only one vehicle so far seems to have had a rather large amount of previous additive that is becoming "unglued". Tested my "black oil" at 9k with oil analysis and found to be excellent. Will be testing my other vehicle at 15k (end of month) with expected same results

I have used high quality synthetic oils with 20,000 mile drain intervals. The engine now has over 260,000 miles and only been opened once to replace head gaskets that were leaking oil on the block.