I,v tryed to get a Total aviation credit card but they just wont answer me , however all of the french airfields I have visited over the last year or so take visa or mastercard ( and some take the BP or shell fuel cards ).
As for the oil ALL the aviation piston engine oils can be mixed but you will start to loose the advantages of the multi grade if you mix it with a "W" oil.
Also DONT USE "W" oils for the first 50 hours with a new engine or cylinders.