Maybe we ought to get back to basics a bit here.
There are essentially two reasons for oiling/greasing the prop tube.
The first is to reduce friction between the bushes at the ends of the tube and the revolving shaft. In most models this really only means the inboard end as the water itself will act as a pretty effective lubricant with most bushes at the outboard end.
The second is to help prevent water finding its way up the tube and entering the boat. Now, with most of the sorts of boats we run it isn't the grease or oil which stops the water although it does help. On the end of your shaft you will have a propeller(!). Inboard of that you should have a lock nut to stop it unscrewing and falling off. Inboard of that you should have a washer of at least the external diameter of the prop tube. This could be brass or nylon etc. When the boat is going ahead the propeller will push the washer up against the prop tube and and effectively seal it which will very effectively stop water getting in. Unless you have some sophisticated engineering setup with internal thrust blocks etc. it is the pressure of the propeller on the washer and thence to the prop tube which actually takes the load and drives the model forward. As it is submerged the water acts as a lubricant.
But when you go astern (which will normally only be a small proportion of the time spent going ahead) the propeller is pulling on the shaft so the load is taken on the nut and washer that you should have installed at the inboard end. if you haven't then the load will be taken on the motor bearings which is maybe not a good idea except at very low power. Now, if there is a lot of 'play' on the shaft; i.e. you can move it up and down appreciably, then going astern will move the outboard washer on the shaft away from the outboard end of the prop tube which may allow water to enter which is when your grease/oil comes in handy. So the best practice is to tighten up both ends of the shaft until it is locked in place and then ease off just sufficiently to allow the shaft to move freely. This will minimise the likelihood of water entering when going astern.
Some water will get in, it is inevitable, even on full size craft so the oil/grease in the tube will help prevent corrosion while the model is laid up at home.
Colin