I was recently refurbishing a microscope and made an adjustable eyepiece (the original eyepiece didn't have any diopter adjustment). I had also cleaned up some of the slides on the 'scope because they were a bit gritty. Since I'd removed the magic sticky grease, they now moved far too easily.
After some searching, I discovered that the correct item to look for is called damping grease. This is rather magical stuff with an extremely high viscosity. It's used whenever free movement needs to be prevented (e.g. in microscope slides!) or when you want to give a smooth feel to a knob or similar (e.g. volume control knobs). Another use I've seen is the little doors on cassette players which pop out slowly - the hinge is usually covered in damping grease to slow it down.
The most readily available brand (at least in the UK) appears to be NyoGel by Nye Lubricants. They have a page with datasheets at http://www.nyelubricants.com/products/nyogel.shtml. The 767A, 774, and 774L greases are available on EBay from user "bigbillgdr" (http://www.ebay.co.uk/usr/bigbillgdr) and come in a convenient syringe. They worked great on the microscope and eyepiece (I can't remember which one I used).
I decided it would be interesting to do a side-by-side comparison of the greases, along with some normal lithium grease and 3-in-1 oil. To do this, I rigged up a 6mm steel pin sliding in an aluminium bushing with a weight on the pin to pull it down. With the pin coated in grease, I timed how long it took to fall through about 30mm and also videoed the action. Here's the result:
Obviously, this is very rough, but it does show pretty well how much thicker the damping greases are. It's very hard to tell beforehand which one is best - I initially tried the 774 (medium) on the eyepiece, but it was too thick so I used 774L (light) instead.
The side-by-side segment of the video was composited in Adobe Premiere, once I figured out how on earth to do it! It's an extremely feature-rich program. I was unable to find a way of adding a timer which counted in 1/100ths of a second, so had to make do with a frame counter instead (you'll notice that the last two digits only count up to 24 and then roll over, since this is a 25fps clip).