@OP
I'd use a near empty tank, and not more than 5% petrol when it's warm outside, but without loading the engine (v<30mph ; no uphill or fast acceleration).
Risks: engine knock, too hot flame, exploding burn (vs slow burning (diesel) )
Personally, I'd use veggie oil or clean engine oil (or GB oil, or anything I dont need.) I usually just dump in the tank everything I dont need, as long as it's unused... (lots of metal in used GB oil for example)
Oh, and pack a spare fuel filter!