ST90 vs SAE30

[warts]

I don't know which one WT#1 used, but automatic transmission fluid (ATF) comes in several flavours. There is one of the later grades that is not so good with brass/bronze bushings. (I've heard).

As just about every newly produced car in the US has an autobox, every oil brand packages ATF in several grades. Cars with engines over 500hp and 500lbsft torque use atf, often sealed for life are not that uncommon. Engines and attendant gearboxes with less than a hundred hp are distinctly rare.
I would think even the most basic one would exceed the loading seen in a Lammy and be perfectly fine.

Wiki: https://en.wikipedia.org/wiki/Automatic ... sion_fluid

Hopefully @Warkton Tornado Number 1 will be along will a solid recommendtion.

[Warkton Tornado No.1]

I don't know which one WT#1 used, but automatic transmission fluid (ATF) comes in several flavours. There is one of the later grades that is not so good with brass/bronze bushings. (I've heard).

As just about every newly produced car in the US has an autobox, every oil brand packages ATF in several grades. Cars with engines over 500hp and 500lbsft torque use atf, often sealed for life are not that uncommon. Engines and attendant gearboxes with less than a hundred hp are distinctly rare.
I would think even the most basic one would exceed the loading seen in a Lammy and be perfectly fine.

Wiki: https://en.wikipedia.org/wiki/Automatic ... sion_fluid

Hopefully @Warkton Tornado Number 1 will be along will a solid recommendtion.

I may be incorrect in saying that colour categorises the type of an ATF, but the type I use is red. However, to be more specific, it is Dexron III.

I bought a good brand of 5 litres for about £25.

Because I initially tried it (without any real confidence!) I used it in a ScootRS six plate clutch that I was nursing due to slipping under load. The ST90 was replaced with no strip down, just the refill with ATF. The clutch no longer slips (Pacemaker with 19 x 46 equivalent overall to GT200) & gear shifting seems easier than before.

[rosscla]

The original spec for Vespa gearbox is SAE30 but when I do recall that there was trend for using ATF in those at one point.

[warts]

Going back to non atf oils, the way gear and engine oils are measured for viscosity - the headline number on the tin - is different. I think (but verify!) sae90 gear oil has the same viscosity as 30wt engine oil.
Tribology is a complex game - the room temp "thickness" has nothing to do with anything.