Stupid question.... is it suitable to top off front and rear fluids with a normal synthetic 75w90? I plan on changing the fluids at 1000, and then switching to a more affordable than grape juice fluid, as it seems like a commonly accepted method.

If your just interested in topping off the fluids for a short time, I dont see much harm in mixing lubes. I wouldnt advise it for the long haul as some may not be compatible with others and could loose some of the lubrication qualities.