Any light oil is good.I use some of my clean car oil and some times a 3-in-1 general purpose oil in a can.
If learnt that with the OB it doesnt hurt to squrt some in there before and after each run.
As for the motor.
brushed... after a run just put it in the sun. If the boat flips and the motor is drowned, then spray electrical contact cleaner in there. Leave in the sun. I dont like to run my brushed (700's or 05's) when thr brushes are wet as they wear out faster.
brushless... leave it after a run and put in the sun. If drowned. WD40, CRC, then electrical contact cleaner.
I have been know to flip my boat in a practice session and turn it back over and keep on going. Just drain the water out of the motor. This hurts the bearings a bit, but i have had a nem10XL on my tunnel now for 15months never bothered to pull it off and it is still going strong. Of course i would not do this to a hacker, nue or any really expensive BL motor. At the end of the day it gets a good spray of WD40 and contact cleaner. You will need to look at replacing the bearings on the brushless every 6 months.
most importantly as kris said - have a good water tight radio box to keep all electronics dry.
David