If there are difficulties in removing the dipstick tube you can remove the fasteners securing the tube to the engine and remove the pan with tube attached. This a fiddly job and if you are not inclined, then leave the pan be. Read the 700/900 FAQ's about flushing your trans. It is a much better way to replace the old fluid than a fill-drain-fill procedure using the pan drain plug.

The Valvoline synthetic is a good choice, assuming it is a type (Dexron?) that meets Volvo's recommendation. Just stick with the big name brands and you are reasonably assured of good quality.

Here's the FAQ link:

https://www.brickboard.com/FAQ/700-900/TransmissionAuto.htm

I have done the flush to a few Volvo automatics. My main comment on the FAQ procedure is that you have to be quite certain of the amount you drain out and then replace it nearly exactly. I don't like the idea of using two people and trying to keep up with the draining fluid "on the fly". I did it in stages....drain into a 2-litre jug and stop the engine. Add 2 litres of new fluid and restart the engine - and thus the flush. Repeat until clean new fluid appears in your clear plastic drain hose. The new fluid does not seem to mix with old in the trans. One second maroon/brown fluid is coming out and the next, it's cranberry red and clear. Stop and check level. If OK, button everything up, go for a minimum 20km drive and check again now that the trans is at full temperature.
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Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), and XC60. Also '77 MGB and four old motorcycles. Long gone: 1981 244, 1994 940 and 1998 S90.