zinc on rudder - motor or gear oil

Patrick McAneny

I have two zincs on my rudder.The one on the starboard side just over half way down seems to be working, slowly being eaten away.The second near the bottom of rudder on port side look almost new.Did the boat come from Amel with two zincs or did a previous owner add the port side one? I just replaced my shaft oil seals and can not remember whether I should use motor oil or gear oil.The placard on the reservoir is in french, the only thing I understand is SAE 20 -40.
Thanks, Pat SM 123