Instead of grease I use a product called "Boshields T-9", available at pretty much any marine store. No oily residue to attract dirt.

Boshields was developed by Boeing to spray on their electrical components on aircraft. Once airborn everything sweats and will corrode.

Boshields is great to spray in the snouts of your sunroof drain pan. It will totally stop the rust and does last quite a while. take a new piece of metal and spray half of it leaving the other half dry. Leave it out in the weather and you'll be amazed at how long the Boshields protective qualities will last.

BTW-I have no affiliation with the Boshields company or Boeing. I'm a boater that has used this product in marine applications with great results.

Cheers,

Bill