My dad changes his oil every 15000 miles with synthetic mobile one oil. He runs duralube, and changes the oil filter every 3000 miles, and tops it off with mobile one. he checks the levels all the time.
so far he has had several cars with extremely high mileage
77 lincoln 350k+
85 chevy caprice 350k+
78 lincoln 300k+
81 cadillac seville 290k
77 cadillac eldorado 300k
(??)bonneville deisel 360k
he had a fleet of chevy/ford/dodge work trucks and delivery vehicles same results. several salesmans cars most of them were so high mileage it wasn't even funny
90 pontiac transport 290k
96 transport 230k
92 bonneville sse 296k
94 chevy high top conversion van 190k
every one of these ran fine when sold or are still running fine now.
transmissions and other drive line components are only good for so long.
My 84 yota had 256k on it when I yanked the engine for a swap. It ran fine I just wanted more power. had a lot of timing chain slap.
Wayne ,my friend, stuck that engine block in his truck. he threw some new tensioners on it . It ran another 30k before he sold the whole truck still running fine. and that engine got dogged all of it's life with big tires and a heavy right foot.
basically I believe in synthetics
consistant oil changes and proper levels are the biggest engine savers though. and of course proper maintenance. gaskets seals and what not.
I just run the synthetics, because of past experience with it, and because I can.
if you can run it sure it will be better for the engine (synthetics don't break down like dino oils) but running an engine on it's side will kill the bearings in a hurry. and oil is like water it won't compress. so if you flop your rig and it stays that way for a while I would suggest you pull the plugs before you restart it and crank it over a few times.
if you've ever seen the side of a block cracked open with the rod hanging out you'll know why I suggest that.
and deep water crossing with out a snorkel especially at higher speeds will bring the same effect.