I removed the transmission cooling fittings from the radiator of my 68 convertible (J-code 302) to inspect and clean them and was wondering what to apply to the threads as a sealant before installing them. Wasn't sure if t-tape can withstand hot transmission fluid or if there is a particular sealant (if any) that the factory used. Maybe a thin coating of Permatex sealant?
If you are concerned about the operating temperature of teflon tape, here is a direct quote from a web inquiry.
The tape is commonly used commercially in applications including pressurized water systems, central heating systems, and air compression equipment. Temperature Range: -268?C to +260?C (-450?F to +500?F). PTFE is completely stable up to +260˚C (500˚F). Decomposition is slow up to 400?C (750?F).
Another inquiry also stated that teflon tape is chemically inert to petrol and oils.
In summation, I think that you are good to go to use teflon tape on your fittings. I've never had any compatibility issues with teflon tape. If your concerned about cosmetics, you can trim off the excess after installing your fittings.
Hope this helps.
Frank