There is no doubt that the Aeroshell 15W 50 oil is some runny stuff.
But, if it runs out of your engine, it may be the engine, as opposed to the oil to blame.
I went to ANC once to pick up a C185. The numbnuts who did the maintenance test flight after float install warned me that that sucker must be using a LOT of oil, cause he had to add four quarts... Groan---I asked him which side of the dipstick he'd used to determine that it was low on oil......long pause. Well, he'd put 14 quarts or more in a 12 quart engine. I drained some oil out, but that poor engine spit oil out of every orifice right up to tbo. THAT engine really didn't like Aeroshell 15W 50.
All our airplanes here run 15W 50, and we do not have oil on the bellys, like ever.....and these trainers get thrashed hard. We operate down to -30 F, though we try not to do multiple landings and takeoffs when its that cold. But these poor engines are constantly doing full power followed by idle, followed by full power....etc.
We've got one engine that's at 2300 now since overhaul, and going great. Run on 15W 50 since new.
I've run probably a dozen engines or more on 15W 50 and never had a problem with excessive oil use.
But, as I noted, if there are any seeps in your engine, the 15W 50 will find them.
As to mixing oils, you can mix ANY approved aviation lubricating oil. They are ALL required to be compatible.
For the record, I now use Exxon Elite 20 W 50 oil in my personal plane. Not because my engine spit 15W 50 out (and yes, I did run it for a few years) but because Exxon has some great sales, with free shipping anywhere in the US (including AK) on their oils at OSH and Sun N Fun.
Aeroshell 15W 50 works fine, but if your engine is prone to leaks, it'll find em. Put it in a tight engine, and it's just an oil like any other.
MTV