Why does my reverse gear need the engine to rev up in order to engage on a 1970 Oldsmobile Cutlass

You very likely have a "can of worms" , depending on what may have been done to the trans. Any disturbing like changing oil, may stir up wear debris which could have worse conditions. Sounds like a servo or a valve is sticking, revving the engine increases pressure from the front pump ( runs at engine speed). The increased pressure operates the servo or valve. Possible some wear debris is causing the servo/valve to hang up. The problem is that an oil change can stir up wear debris. My last Olds was a '79 Cutlass ; In about 85 , an acquaintance who ran an independent trans shop advise me not to change trans oil as it may stir up wear debris. GM trans of that era generate a lot of debris ; I have cleaned a few ,starting with a putty knife in the bottom of the pan. I had a few turbo 400s in Buicks and one Pontiac - I can't remember which ones I cleaned. My reference for clean is an 85 Nissan 300 ZX, when I dropped the pan it was clean , a wipe with a paper towel and it shined. Assuming your trans has not been serviced , I would just drive it And not touch it. I congratulate you for having a '70 that runs. If you were in the US and loved the car you could consider having an independent shop rebuild the transmission.