Since the joints have to be very tight to guarantee for a good seal, they have to be properly lubricated with high-vacuum grease (Dow-Corning, low vapor pressure, stiff, nonmelting silicone lubricant maintains its consistency from T=-40° to T=+204°C).
Both connecting joints also have to be cleaned properly before being lubricated. A small amount on the top part of the male joint is usually sufficient (If the male joint is rotated inside the female joint, and the top part is clear all the way around, enough grease was applied). Too much grease does not necessarily provide a better seal. It often ends up in the product because it dissolves relatively well in most organic solvents i.e., diethyl ether, THF, hydrocarbons, etc. It also can cause many problems in the purification of the crude product as well, particularly if the product is a solid. Its exposure to the solvent has to be minimized as much as possible. Therefore, it has to be used sparingly and in the right places.