I removed my exhaust manifold (55hp diesel) the other day to track down a problem and now will replace it. Is there any goop, grease or other lubricant that I should put on the gasket before putting back on?
No, just a new gasket if the old one is totally compressed, might still seal well but new ones aren't that expensive. Your EGTs will burn off anything you put on there.
I used CRC ManisealExhaust Cement to enhance the gasket. I bought it in Australia but believe a similar product will be available globally. It is commonly used in car engines, and is designed to take exhaust temperatures.
If it is thick caked on stuff use a screwdriver to scrape it off, lighter deposits can be removed with any number of solvents, lacquer thinner, brake cleaner etc. Use the solvent for the mating surfaces.
Just yesterday I ordered this material to make a custom gasket for a new turbo setup that I couldn't find the proper gaskets for. Don't know how it will work yet but it is cheap and has good reviews.