PISTON 3276405 - Caterpillar

has always recommended wetting most O-ring seals with the fluid that is being sealed when the seals are installed on the parts. This was generally a good practice, but resulted in unclear recommendations for other seals, such as those used on air pipes. Some of these seals, and also some in the cooling systems, will swell or distort if they are covered with a petroleum based lubricant and will actually cause leaks.
The seal color cannot be used as a guide for the correct wetting agent. For example, some black seals are of a Buna-N material and can be used with most petroleum products. However, black seals are also made from EPDM material and these seals will swell from contact with oil or fuel.
The best recommendation for O-ring seals in most applications is to put glycerine on the seals at the time of installation. Glycerine does not damage any known seals or cause them to swell.
There are some exceptions to this practice and you should read the applicable Service Manual if in doubt. For example, the filler band seal (not really an O-ring) around cylinder liners should be coated with oil to provide intentional swelling after installation.