Sure, that is what I do when I am on the road. If I am on reserve I put oil in for three gallons, add three gallons fuel, then figure the rest out from the space I have left. You've got to assume there is properly mixed fuel/oil in the carb bowls, fuel line, etc. Most 2T oil made for pre-mix on the street mixes very quickly with fuel. A good shake of the bike after filling couldn't hurt though.
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Andre E. Benguerel
1996 Honda NSR250R
Daijiro Kato/Fortuna Replica #74

2000 Honda RS250R (NX5)
2008 Honda RS250R (NXA)
AFM Expert #430