Generally, it is not recommended to grease the ground glass joint holding your flask when using a rotary evaporator. Organic solvents that are commonly being evaporated readily dissolve silicone based grease causing it to end up in the product. The likelihood of grease contamination varies per type of solvent. Polar solvents (THF, diethyl ether, DME, dioxane) are not too bad as grease will hardly dissolve in them. Non-polar hydrocarbon solvents such as pentane, hexane, and petroleum ether are worse as they easily dissolve grease. Halogenated solvents like DCM are well known for stripping grease out of joints and are by far the worst.
In most cases the lack of grease will not influence the vacuum quality as pressures down to 20 mbar can be achieved without it. Sometimes lack of lubrication can cause your flask to get stuck but most rotary evaporators have special clamps to get it undone again.
There are, however, cases in which greasing your flask will be beneficial. High boiling solvents such as toluene require a higher vacuum to be removed. Here, grease is needed to improve the seal and allow for the pressures required to remove that solvent. Just make sure to clean it for the next user!
Similarly, poorly functioning vacuum systems might require a better seal to obtain pressures needed to remove lower boiling solvents and will also benefit from greased glass joints.
Overall, greasing your ground glass joint when using the rotary evaporator should be avoided to prevent contamination of your product with grease. If the vacuum needs to be improved and grease is necessary make sure to check the compatibility of your solvent with the grease. In these cases a better solution can be to use hydrocarbon or fluoroether based greases or inert Teflon joint sleeves to avoid contamination of your product.
From left to right: examples of silicone, hydrocarbon and perfluoroether based vacuum greases and Teflon joint sleeves
