WE HAVE several "candle or oil" hanukiyot (cHanukah menorahs).
For the last several years we have been using oil for one.
Each year I struggle to clean the plastic cups of the previous night's black smidges.
Since I fill the cups to the brim, the flame lasts not 30 minutes or an hour, but 3 hours or more.
To shorten the burn time, this year I added a little water before adding the oil and wicks.
Voila - actually מצאתי would be more appropriate - the wicks burned more than the minimum time (last night we had a two-hour burn) and there is no smudge on the cups!
Since oil (we use EVOO but any oil will do) and water don't mix, the oil floats on top of the water and the wick-in-a-cork-base floats on the oil.
When the oil is consumed without a water base, a last gasp of the wick creates a smudge on the cup.
BUT, if the oil is consumed and the wick floats on water, there is no "last gasp" to stay lit and there likewise is no smudge.
It's perfectly OK to limit the wicks' (candle or floating-on-oil) burn time providing the flame lasts at least 30 minutes (except on erev Shabat when it must last about 45 minutes - see your rabbinical authority for specifics).
I happen to take the mitzvah to enjoy the light seriously so a two-hour burn is fine with me; it beats any "Hanukah" candle except perhaps a "hurricane" candle. (Tea lights also can last beyond the required 30 or 45 minutes.)
Bottom line: After all these years of lighting hanukiyot I FINALLY "discovered" how to keep an oil cup clean.
By the way, if you decide to use oil, buy and fit the cups at the same time you buy the Hanukiyah. Hanukiyot made pimarily for wax generally have the holes (for the candles) too close together for oil cups.
