Metal in the oil
I would not go past 50 hours without changing the oil when running Aeroshell W15W50.
The reason for this is that the mineral part of this semi synthetic oil can no longer hold in suspension the lead blow by from the fuel. This results in the lead being sentrifuged out of the oil into the oil gallery's in the crankshaft reducing the diameter of the oil ways.
This is the reason Mobil 1 was withdrawn from the aviation market.
This is not a reason to not use Areoshell W15W50, it is a very good oil but like all things it has it's limitations, I run all four of my aircraft on W15W50 and have had no problems as long as the 50 hour oil change is done within the 50 hours.