Transmission fluid compatibility
 

Hi
In question is the 850 non-turbo automatic, maintenance unknown, tranny shifts rather hard. Odometer stopped at 150,000. The fluid is brownish but not too burnt smelling.
My question is about changing the fluid. I have read that doing three or four 'drain and fills' over a period of a couple weeks might be better that just flushing on these cars. I am most concerned about mixing different types of fluid- I know Dextron III is ok, but what if someone put some fancy new synthetic in there?
Can we damage it further mixing fluids? Are there completely incompatible tranny fluids that might be used on 850 Volvos? Is it possible to tell by looking or is there some test? I am intimidated about creating a problem!
Thanks!