Replacement Buna O-Rings are available for all of the signature Pro-werks Easy Turn Fill Caps. Be sure to check out the entire line up - Billet aluminum Easy Turn Fill Caps are a Pro-Werks specialty. Frequently Asked Questions Q: Is Buna rubber compatible with my fluid?A: Buna is a synthetic rubber compound suitable for use with many fluids. Buna components may need to be replaced by a material compatible with the specific chemistry of some fluids. Q: What should I use to lubricate the Buna O-ring?A: Applied periodically, a light coating of Dielectric grease has proven to work well as an O-ring lubricant. Q: How much do I need to tighten a fill cap?A: It is not necessary to bottom the fill cap into the bung−just adequately seat the O-ring. The radial squeeze action on the O-ring creates a positive seal and helps prevent the cap from coming loose.