Some engines have the ability to retain fuel (alchohol) after you run them dry, unless it is really hot. The problem with glow fuel is that the methanol fuel absorbs ambient moisture, so if there is any at all left laying in the bottom of the engine it will saturate itself with ambient moisture and leave rust causing water inside the engine.

Since fuel is drawn through the rear bearing, the rear bearing is the most susceptable (two stroke engine of course) to rusting. It is strongly suggested that you apply after-run oil after you think you have gotten all of the fuel out of the engine, then leave the glow igniter on and hit it with the starter again. You may be surprised that the engine will fire and spin a little. This is because the AR oil has displaced some of the fuel laying around and pushed it on up the ports from the crankcase into the cylinder. If you do this a couple of times you won't have to worry about replacing bearings.

If you are tight on money, you can go to either Pep Boys or another auto supply place and buy some Risolone and air tool oil and mix them about 50/50. These two oils have great coating and rust inhibiters. Like the gent said before, you'll have enough to last a life time. Four strokes are not as susceptable to this, since their fuel mixture/airflow takes a different route. They are also much more difficult to get oil into, since the carb is on the back. Most of us put a piece of fuel tubing on the crankcase vent and put oil into them through it. This is often the only way, when the engine is enclosed in a cowl.

Happy landings.