I would try their 30% heli fuel. Even though it's only 15% oil, it'll probably work fine.

4 strokes need more oil, mostly because they're ringed engines, but also because they need to lubricate the cam, lifters, rocker arms, and valve guides. The FS26-C has a bushed camshaft as well. Yes, more oil will make it smoke like crazy, but if you're running outdoors it won't be too bad. If you can get a low smoke heli fuel, that seems to be the best way to go.