You have probably asked the two questions which will get as many different responses as there are people on the board. You can find many opinions in the archives and take them for what they are worth. My experience is this. I have two 850's and an S70. All three were purchased used with the low mileage of 75,000 to a high of 112,000 when purchased. In all three, the engine oil has been replaced with synthetic (Valvoline Full Synthetic 5w-30 in the 850's and Mobil One 5w-30 in the S70) and all three have had the transmission fluid replaced with Mobil One Synthetic Transmission Fluid. I have had no adverse effects (leaks seem to be the common "problem" you find on the board) and did a full flush on them. If you check, there are posts on this board about changing, or check Bay 13 at Volvospeed. Basically, you carefully remove the top transmission hose going into the radiator and stick 1/2" clear plastic tubing (long enough to reach a container like a milk jug on the ground) on the radiator outlet. Start the car and let two quarts pump out. Shut the car off and add two quarts. Start it up, pump out two more. Add two quarts. Keep up this process until the transmission fluid being pumped out is the same color (cherry red) as the fluid you are adding. Then reconnect the transmission hose to the radiator and top up but do not overfill. The consensus seems to be that changing the tranny fluid every 30,000 miles is a good idea. In any case, frequently check the fluid for color and smell. Dark, burnt smell is bad. This process is easy and dropping the pan and draining doesn't get the fluid in the torque converter. Have fun!