It was probably silicone grease. If you are going to upgrade the electric bits, a "real" prop shaft assembly would be a good idea to move the comversion on from toy to model.
Instruction sheet here -
https://fccid.io/CVTRP6400H/User-Man...-Manual-115921
I didn't notice any mention of lubing the shaft, but did see that it was reccomended to empty the water out after sailing.