Garages tend to advise against replacing transmission fluid in an older car because doing so can cause problems for which they don’t want to be held responsible. I think Volvo changed their advice to leave the fluid for the life of the transmission unless towing or driving in a harsh climate. What they didn’t say is that the transmission probably wouldn’t live as long without regular fluid changes.
Your car probably has an AW71 transmission as it’s a turbo model. Does it have an overdrive switch? This is a tougher transmission than the ZF box used on non-turbo cars of the vintage. Either would probably benefit from a fluid change if the fluid is dark and smells burnt.
The most popular method is gradually to drain and mix in new fluid over a longish period until you’ve done this often enough to have nice clean fluid. This is less of a shock to the transmission and helps gradually to lift dirt out of crevices.
On my 940 I just went straight in and changed the fluid in one session using the gibbons (or cooler line) method with no ill effects. I didn’t drop the pan or change the internal filter as others have said it’s rare to see one that dirty but if you want to do a proper job that might be worth considering.
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