Is the acrylic used in the MSA different than the acrylic used in the polymer varnish? I am just wondering why I prefer the MSA's appearance over the polymer. The MSA looks more like Damar.
Yes, the MSA is a different acrylic than the Polymer Varnish. Keep in mind we choose the resins we actually use for GOLDEN products from literally hundreds of choices. The dozen or so that meet our demands have been tested extensively. Polymer Varnish is not as water clear as the MSA Varnish because of its refractive index. It's a good product but the MSA is clearer and will produce a higher gloss.

If it is not a good idea to put acrylic over oil, then why is it OK to put the MSA (acrylic solvent soluble) over oil paintings? Once the solvent evaporates it is still acrylic over oil.
MSA resins are completely different than any waterborne resins used for acrylic paints. The film is much tighter and the solvent helps it attach to the surface of the oil paint, provided it has been well cured before the varnish has been applied. If a surface is too oily or slick, the MSA cannot make a sufficient bond. Waterborne acrylics have a natural aversion to the oil paint surface. However, if it was well cured, and sufficiently greesless and abraded, you could get waterbased products to adhere to the surface. But as a rule, we tell people not to do it, because there has not been enough long term studies conducted to prove the acrylics will adhere over a long period of time. MSA resins were developed in the late 40's and have been thoroughly tested and approved for use by conservation labs around the world.