Not alike at all in my opinion The cluster gear acts as a reducer in all gears but direct and then in high gear, the sliders are handling the power in conjunction with the input shaft and output shafts.
The ring and pinion gears are being fed power through the driveshaft and are loaded anytime the car is moving forcing the gear oil in and out of between the gears over and over, trust the makers to use the cheapest lube that they can that will do the job and not cause part failures and customer complaints.
A friend, and fellow stocker class drag racer, of mine tried to use 30 Wt. motor oil in his 8 3/4 for one pass at the track, it was very noisy on the return road after that single pass so he took it home and put in another new set of rear gears