I would use oil since oil and fuel are basically the same thing. A tiny smear of oil on the o-ring will safely dissolve in to the fuel and be burnt off in normal combustion.
Say a fingertip of oil on the o-ring is 1ml, dissolved in to half a tank of fuel thats a ratio of 50,000:1 which is totally insignificant, similar to what is coming in as vapour from the crankcase breather.
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