The breather nipple's main purpose is to let the crankcase breath. A small amount of oil is forced by the pushrods and the piston on each stroke and into the lower crankcase. I have put an empty small tank on a 1.20 4 stroke and, after the flight was over, had about two to three ounces of oil in the tank. Do not restrict this nipple. It is also the best place to inject your after run oil at the end of the day. Inject about a teaspoon full at the end of the flying session and hold the prop shaft down and rotate the engine to get a good dispersion of the after run oil around the thrust bearing. It is sometimes hard to get the oil in but; if you connect a squeeze bottle to the nipple and (with the nipple higher than the rest of the engine), turn the engine over while connected to the squeeze bottle and you can suck some in. Now, it will also spit out and all over everything when you turn the engine over with the nipple positioned low so put a paper towel or rag over the nipple while stored or when you are turning the engine over by hand. When flying, it is a good idea to have a short piece of fuel line connected to this and exiting the fuse in a convenient place to keep from spraying oil into the engine bay.