As soon as my Mazda 3 was out of warranty, I switched from the 7,500 change interval, to 15,000 miles with the Mobile-1 that is advertised as such. Last oil change I did was at 330,000 miles and the odometer reads just over 340,000. I don't add oil between changes and it actually gets better gas mileage than the window sticker suggests. Last fill-up was 38 mpg and usually 35-36 is the norm, with a mixture of city/highway. 5 speed 2.0 non turbo engine. The only leak I have, is around one of the half-shaft seals coming out of the transmission, so I've got a few drips of gear oil where I park. No engine oil leaking at all.
When Mobile-1 was brought to my attention, I started using it on some of the old junk cars I drove in college without any problems, and greatly stretched the recommended change intervals. Japanese engines, so maybe that had something to do with it. Took an old Datsun 510 with 100K miles on it, and put another 100k miles on it, before lack of A/C (and getting a job out of college) let me buy something new. Not the slightest idea what oil had been run it it, before I got the car.
Oil, of any brand, is (or should be) a LOT better than it was. Advances in refining brought 'conventional' much closer to the consistency of synthetic, in the 1990s. Hence the car manufacturers have been stretching out the change intervals. When I started changing oil, on 1970s vehicles that my family owned, 3,000 miles was a common change interval, and you could really see the difference in consistency when the pan was emptied, vs. what came out of the can. Way thicker / sludgier coming out, even after 'just' 3,000 miles, and adding oil between changes was certainly a common thing. When I change the oil in my car, it runs out about as thin as the clean oil that comes out of the container to refill it.
Thinner viscosity oil takes less effort for the pump to move it around in the engine. Nobody in their right mind would have run a car on 0-20 or such in the 1970s, unless you lived north of the Arctic circle.... but my 97 ranger manual specifies 5-30, and my Mazda 3 is even thinner than that.
The truck has a 5,000 mile change interval. I have no doubt I could stretch that to 15,000 miles, except that I drive the truck so seldom that it might never get changed again
Rear main seal leaking - best switch over to valvoline Max-Life. That stuff really works. If that won't fix it, you're gonna have to put up with the leak, or change the main seal(s). Had to use it in my 68, after a disastrous switch to Mobile-1 left the 289 leaking like a screen door on a submarine. After the change to max-life, no more oil leaks. You could probably mix 50/50, the max-life with some other brand, and at least get some of the benefits.
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