There have been a few posts lately, criticizing the factory recommendation
for using ATF is the early manual gearboxes. The criticism is unwarranted.
Many gearbox manufacturers recommend ATF. Consider that ATF has about the same viscosity as 10W motor oil, and is only slightly lighter than 75W hypoid oil. ATF has a lot of detergent in it for prevention of varnish in the auto trans hydraulics, and is more than suitable for all but the most severe duty in manual gearboxes. The only place it should not be considered is in the differential, where the high shear loads can cause it to break down quickly. Meanwhile, it can be used in the early boxes safely, and may help with shifting 'notchiness' problems associated with using oil that doesn't flow well when cold. If you have a big concern, invest in a few bottles of synthetic ATF.
In the mid-80's, one of my toys was a Saab turbo, equipped with a dr bob-improved engine management system. The enhancements were in the areas of more precise fuel and boost management, so it was in the 200hp range.
Meanwhile, the gearbox was a bit balky when cold. The factory recommendation on gearcase oil was 10W-30 motor oil. I was concerned about the notchiness from first to second as the car was warming up, and also concerned about the extra strain that 50% more torque was placing on the gearbox. I had a sponsorship deal with Bel-Ray at the time, for my motorcycle-engined LeGrand sports racer. The Bel-Ray gurus suggested that ATF would be a very valid choice for that application, followed by their 75W-90, or perhaps 85W synthetic gear oil only if the car wasn't driven in very cold climates. Since the car was doing the commuting duty between Los Angeles and Mammoth for winter ski weekends, I tried the ATF. The cold-shifting problems disappeared. I soon switched to the 75W-90 after a warning from their engineers about the shared use in the differential under the extreme loads, and the problems stayed away.
So-- got a gearbox that's mated well to the capacity of the engine? Is the gearbox oil separate from the hypoid gears in the differential? If yes to both of these questions, a good quality ATF might solve cold shifting problems. Otherwise, the Bel-Ray 75W-90 oil worked perfectly, especially since it was a free benefit to me. Too bad I couldn't stretch the tire deals to cover the street rides... I suspect that the Redline and the Bel-Ray gear oil products are similar. Bel-Ray is commonly available at better motorcycle performance parts places. Motorcycles put a lot more strain on gearcase lubrication than our cars do, so I have no problem recommending it for use in the early cars.