[bestgenman]

Most of these fans are "muffin fans" (the type used to cool electronics) and can be lubricated but you must remove covers and in some cases floor plates (like the bottom of a cabinet). They are meant to be lifetime lubricated but like us, lifetime lubrication some time wears out early (like about 55 or so). Some are squirrel cage design and somewhat harder to get to.
It is a wise idea to inspect and clean the coils at the same time as dust build up inhibits heating efficiency.