From what I can establish, the "W" rating on gear oils (which supposedly defines the low temperature "winter" viscosity) is actually based on the temperature at which a defined viscosity occurs. As such it will also impact the curve of viscosity against temperature, but because the changes are very rapid at the low temperature end of the graph, the real-world impact of going from 85W to 80W is probably fairly minimal other than at temperatures towards freezing point and below.
In reality, the rate of change in viscosity is very rapid at low temperatures, and then starts to stabilise. So an 80W/90 gear oil may be 150,000cP at -26C, 150cP at 40C and 15cP at 100C. So apart from a gearbox being driven from cold in low temperatures, the "W" figure is largely meaningless. However because of the difficulty of making mineral oil behave this way (and remain stable under shear), most gear oils below 80W/x are synthetic or semi synthetic.
To me, what seems important is the 40C and 100C viscosity, as these figures are more relevant to my usage. I am far more likely to drive a Series LR in the summer, when the ambient temperature is >25C.
40C and 100C viscosities are below:
Castrol EP90 GL4 - 178 / 16.4 (not available in Australia)
Penrite 80W/90 GL5 - 152 / 15
Penrite Mild EP GL4 - 320 /24.2
Redline MT90 75W/90 GL4 - 90 / 15.6
Nulon 80W/90 GL5 - 137 / 14.5
As expected, the multigrade oils are all thinner at high temperatures than straight EP90. The Widman document suggests this is because they are not true multigrades with viscosity modifiers, but are simply blended towards the lower end of the acceptable high temperature range, to ensure they also meet the low temperature requirements. It is also interesting that the Redline MT90, whilst thinner at low temperatures is actually the closest to EP90 at 100C. The viscosity specification for SAE90 gear oil is 13.5-18.5 at 100C, SAE85 is 11-13.5, so the Nulon product is actually at the low end of SAE90.
Unless the viscosity/temperature curves differ radically, it seems likely that at any given operating temperature the viscosities of the non-synthetic oils will remain in the same relative order, so the Nulon 80w/90 will always be the thinnest, and all the multigrades will be thinner than EP90.
Penrite Mild EP, at 24.2cSt, is actually just inside SAE140 (24 to 32.5), even though they classify it as SAE110, but both Penrite and Roamerdrive recommend it. The reality is that this just means it has the same viscosity as the other oils do when they're a few degrees cooler!
Personally I'd prefer to avoid any possible GL5/yellow metal issues, so I'm going GL4. Whilst Mild EP is, on paper, outside "EP90" specification, to me the downside of a slightly stiffer gear change for a few minutes on a cold morning is outweighed by the reduced noise (and Roamerdrive recommendation) the rest of the time. Given the time, effort and expense of restoring a classic car, I'm willing to splash out on the ha'p'orth of tar....
