Hi,
For what it is worth, here is an experience I had using M1 oil in a car...
Car had about 75K miles on it... Switched from "normal" oil to synthetic, Mobile 1, (M1). After about 300 miles the car began leaking oil like a sieve... Dumped about a quart in the first 300 miles... I did a quick change and went back to "normal" oil, and the problem continued for the next 15K miles, then tapered off to about a quart every 3K miles... Prior to this the car was not leaking oil. It is still leaking a bit more than it ever did, but the mileage is now pushing around 125K. It needs a half quart about every 3000 miles now... The engine has always had oil changes on a correct schedule, and in most cases more often. It was leaking no oil prior to the use of M1.
I have heard this wife's tail about synthetic oil leaking from a car as being the norm if it is above about 40K... I suspect it is the detergent in the M1 which cleaned things up really well, thus leaving lots of little gaps that engine gunk was clogging, thus allowing the M1 to escape...
This is a guess on my part, but it seems to fit the data... I have no idea how it will react in a newer generator engine, but thought I would pass this along... Hopefully someone who knows more than I about cars/generators, and oil will pipe up and address this question...
The valve guides/seals and piston rings in a car engine wear from day one. Makers of conventional oils use additives that take this into account and provide protection for it. Once you switch to a synthetic oil, however, these additives are washed away by the different additives of that oil and can never be fully restored--even if you switch back to conventional oil. True, the conventional oil additives will start to redeposit the additives that maintain the seal effect, but can never fully restore them.
I've got an eighteen year old car that uses oil too--a simple thing like going to the next heavier grade of oil (from a 5W-20 to a 10W-30) reduced that to a tolerable level. Instead of a quart every 800 to 1000 miles or so, it's at least doubled. Now I only have to add one quart of oil between changes instead of two or three. I change oil every 3500 to 4000 miles because the majority of my driving with that car is in city.