Well compared to a sailplane, messy, compared to a equivalent nitro, spotless.
I have a Waco Bipe I had a Saito 170R3 in, the oil just ran off it at the end of the day. I put a MT-57 in it, twice the power, and at the end of the day if I wipe it with a clean cloth you can just tell you wipe something with it. I run 32:1 all the time, if you used 100:1 you might get that after a month. The difference between nitro an gas is like night and day, remember in a nitro engine the oil goes through the engine and out the exhaust, a gasser the oil is burnt.