I recently acquired a really nice aluminum Impempe from a member of this board and I thought that I would spruce it up a bit by using some polish on it. I spoke to a local brass instrument repairer and he recommended a product called Maas creme paste. I must admit that this product took a lot more “crud” off of the whistle than I ever imagined was on it, but now the whistle is slippery to play and I’m having to “kung-fu death grip” it. What, if any, metal polishing products do you recommend or warn against?
I wouldn’t use any product without talking to the maker first.
But, I like a nice patina that shows uses.
You know where the whistle is dull and the tone hole shine.
Very good advice, indeed! I traded emails with Ian before my experiment and he simply recommended “common sense”, but perhaps he overestimated me - I was once told that I didn’t have the common sense that God gave a teaspoon… 
I don’t use any polish on my whistles. I prefer a matt finish for better grip and look. I finish my whistles using 1200 grit wet-n-dry paper, by hand. Small scratches, which are unavoidable using a whistle, can be removed using such very fine sanding paper. Wash well using washing up liquid after sanding, and rinse.
I use just plain ol’ tooth_paste_ (not the gel) on those rare occasions when polishing brass or nickel silver. Patina is preferred though, and usually alcohol and water are the only things used, just to keep the funk off. 
I usually don’t polish my whistles - the brass ones have earned a nice patina to them and the nickle ones just “are”. But, this whistle is very pretty and nearly perfect to the eye so I thought that I would touch it up just a bit. Perhaps it was also a way of making it “mine” since it is my first second-hand instrument purchase… 
I have learned to like the Maas polish creme and would recommend it. The problem that I was having with slipperiness was because I had not removed all of the polish - it takes a lot of buffing to completely remove it.
You’re not alone in having used Maas polish. It does work well. It does need to be totally removed and that requires a lot of polishing. (I found this out the same way you did – the hard way! 
) You might also consider rinsing the whistle out (if possible) to remove any residue that might have fallen through the holes. That one tube of Maas will last you along time … take this from someone who doesn’t care for patina and polishes several whistles semi-regularly.
Judy