ELECTRIC 30-cup perc; best way to remove oil buildups--not hard water stains?

Have three 30-cup electric percolators & want to do a really good heavy-duty cleaning to remove any residual coffee oils. (Yes, have already been over them with soap & water.) I'm talking about the oily filmy "grundge" that can build up inside---ppl who work in restaurants know the type I mean. The problem is NOT in removing any hard water scales/buildup....that's minimal.

Anybody got a good and easy way to do this, especially using cleaners I may have at home (rather than buying some commercial chemical stuff)? Remember, these are PERCOLATORS, not the Mr. Coffee-type pour-throughs. Thanks a million!