Tony, that's a rather definitive statement you make about synthetic causing leaks!

My experience with changing a total of 7 cars and 2 motorcycles in the last 10 yrs over to Mobil-1 synthetic at mileages ranging from nearly brand new to 250k, differs; no new leaks and no increase in any slight leaks that were already there. (5W30 in the cars, 15W50 in the bikes)

IMO, fear of leaks should be off one's list of negatives when considering a change to synthetics. On the list should be the cost, period. I make the cost work by stretching out oil changes to about 10k miles, with my mostly highway use. This reduces the cost of filters too, not to mention reducing my time under the cars. It also results in less used oil and filters being disposed of or recycled.

I have owners manuals for 1981, 1983, and 1985 240 cars and my 1998 S90. In each book, Volvo recommend 7500mi (or six months) oil change intervals in normal service. For those old turbo engines they recommended 3750 miles. And that is with the conventional oils of the day.

Conventional engine oils have improved a lot in 20 years, particularly in the ability to handle heat without breaking down. The performance gap between them and synthetics has narrowed.

But if I had a red block turbo, I would probably use Shell Rotella T6, a 5W40 grade full synthetic that gets called a "diesel oil" but which meets the latest API-SM spec for gasoline engines. It's a lot cheaper than Mobil-1.
--
Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.