I've consistently drain my manual 850 and 70 series at 50,000 to 75,000 miles and used Redline MTL synthetic. It takes about 2 liters and there are none of the flush difficulties that we go through on the automatics. Remove both plugs on the drivers sides...the bottom is the drain plug, the top is the fill plug. Because of the location of the drain plug it will spill onto the subframe cross member and 'spash at bit, so put some protection around your catch pan....cuz some will miss. I've never had any transmission synchro failures and decided to change the fluid on my cars after reading about the second gear synchro going for those who follow the recommended maintenance which is "nothing".
I'd read about Redline about 7 years ago and have used it consistently. Once I didn't have it and used Mobil synthetic gear oil. It seemed fine, but like the redline because it's pink and if you have the car long enough to change it again....gives you a "visual" of the change in the fluid. Which may mean nothing. The Mobil was brown/tan.
While I would not use ATF, if you were stranded and needed something...better to use ATF than motor oil. The Moter oil is too slippery, will coat your synchros, they won't work properly and then I have no idea what to do.
One more tip...to get in whatever fluid you are using...you can put a long clear plastic hose (it's good to see the fluid passing down) on the end of a funnel and fish the hose down into the fill hole from the master cylinder are of the engine compartment.