I use vertical set up, mostly a microscope for my amber pictures. The deeper into the amber the insect is the larger the problems. I have a post with pictures that shows the differens between water, glycerol and oil;
http://www.photomacrography.net/forum/v ... 565#163565
Glycerol or microscope oil can be verry usefull. I have one badly polished piece of amber that is more or less impossible to examine witout a drop of glycerol or microscope oil. But I am not sure If there are any long term effects from microscope oil on amber.
Do not use oil or anything else if the amber surface is not intact. One piece of amber that I have is so well polished that one of the leggs goes through the surface. I will not use any type of liquid on that.
My suggestion is that you use a lens with a lower NA, and a drop of glycerol or microscope oil to even out any scratches on the amber surface.
Best regards
Jörgen Hellberg