In reply to the original questions
1) It's absolutely OK to use 85-90 instead of plain 90. 85 is how the oil behaves when the diff is cold, 90 is how it behaves when it's warmed up. As long as the grade ends in 90 it's OK.
2) Synthetic oil, whether partial or full, will definitely not harm any seals or other components. Moreover, synthetic oil will keep its consistency longer. In simple terms, think of full synthetic oil as being manufactured to consist of many molecules of the same size. Then think of mineral oil as being composed of molecules of varying sizes which on average are the right size for the desired viscosity. Over time the bigger molecules are the ones most likely to get thrashed, which is why mineral oil doesn't keep its properties as well as synthetic oil.
OK, the industrial chemists amongst you will turn pale at my simple explanation but I am confident that no harm arises when synthetic oil replaces or is mixed with mineral oil. It's still oil, just purer if you like.