I'd clean it off when i have done the oil pressure sensor.
When i did mine i used 'gunk' engine degreaser and finished off with TFR(traffic film remover) and the jetwash.
You dont need any gaskets to just change the oil pressure sensor.
Also,i'd get breakdown recovery for your Europe trip just in case the base gasket does let go big time.I was just about to board the ferry home when my oil pressure sensor went and it was about 3-500miles that i noticed the weeping from the base gasket so i changed it.
I aint no mechanic so if others say otherwise,use their advice.