Piston engine has more surface to surface interfaces than a jet engine per energy transferred. Piston engine has less oil too per the output in kW. It has less oil in the sump per output.
Oil in a good gas turbine really SHOULD have an easy life compared to the piston engine, less fuel contamination and hence less degradation.
Most aircraft engines are air cooled just as the jet engine is. If you check out the T972 QF32 investigation thread we go into some detail about oil temps on jet engines.
If you are interested check out "tribology in aerospace gas turbines".