You’ll see multiple recommendations for the gearbox oil. It is almost a religion for some folks.
The series V owner’s handbook page 24 indicates to use Shell Super Motor Oil across all temperatures. Based on some Internet research, Shell Super Motor Oil was a 10W/40 detergent oil.
Many people have used straight 30 non-detergent for years and recommend that. I have a specification plate for an earlier Alpine (believe it was a series 2) that indicates to use Shell "X-100 30" for greater than -10 degrees. I suspect the earlier Alpine spec for straight 30 carried over for some people, even though the series V manual indicates to use Shell Super Motor Oil.
Others have used 20W/50 (detergent) for years and recommend that.
I bought a series V overdrive unit from the UK overdrive gearbox experts in 2013 and their recommendation to me was to use 20W/50. They indicated using straight 30 is also acceptable. I’ve been using a conventional 20W/50 oil in my OD gearbox.
Some people like synthetic oils for the gearbox, but report the synthetics do weep out the seals.
Mike