New gearoils

There are probably at least two reasons for a trend toward lower viscosity numbers in gear oils.
The first being improved fuel economy. Lighter oils require less energy to overcome the internal friction in the oil and result in better fuel economy which may come at the cost of slightly reduced gear life. But how often do you hear of differentials wearing out before the engines or the rest of the vehicle. So it may not matter that much.
The second reason could be the use of greatly improved base stocks with higher viscosity indexes allowing the use of lower viscosity oils which at operating temperatures may show an increase in film thickness compared to the base stocks used 10 to 20 years ago.

Ok, I admit, lowering the viscosity will result in less friction but I'm not a believer in fuel economy oils. it's getting nice results in a laboratory, but in every day practice with the ignorance of the 80% of the consumers (speaking for Holland, so no offense!), it doesn't work very 'economous'.

The techniques in the gearboxes self are also changing. A lot of manufacturers are offering semi-automatic or redesigned manual gearboxes (with automated gearshifting). 6-speed is getting very common now overhere.
Besides the mentioned causes by BuckHntr, I also suspect they need to go to a lower viscosity grade because of the shifting mechanism which can't operate as smoothly and efficient at a 75 or 80 level. Sure, shifting can be done at low temperature with a 75W/80W, but the consumers nowadays expect fast and smooth shifting when they choose for such a transmission.

Many of the synchronizers are getting more conical for better performance, requiring thinner oils do displace, many going the route of ATF for the manual transmission. A lot has to do with the base oils, as mentioned earlier
a multigrades use VI imporvers that could shear. Today less VI improvers are neeeded to get this range of viscosity.

Toyota was using ATF in the beginning of the 90's for their MT's, so that can't be a reason for the sudden change. And sure, if you have the best (or better) base-oil, less additives are required, which is always better, because adding additives is always a compromise.

SAE 70 in gear oil equates to an SAE 5 in engine oil. It looks like a sythetic poly glycol similar to ATF III (SAE 10). I wouldn't worry too much about 70W not registering yet - there probably just hasn't been a requirement for it in gear oil yet.

I agree though - what a headache! Too many technologies out there now!