I've been using Magnum fuels since I moved to Wichita. The owner of the LHS had testing done on Magnum and other fuels, and Magnum came back with just as much oil as others. I've put 6 gallons through my ST 40 and have not had a bit of fuel related problems with it. As far as using Magnum (12% & 15%), its been just fine for me. And he doesn't carry it just to make more profit - he mark his fuel up only $1.00 (he's shown me his invoices for the fuel). A lot of the guys in the club use Magnum and have no issues with it.

If in doubt of the oil content of any fuel, you could always add some caster oil just to be safe.

Hogflyer