On 13/08/2020 09:36, Andy Burns wrote:
Rob Graham wrote:

this oil "BP Energol GR 425- EP"

I can't find any mention of this oil or an equivalent

BP/Castrol say Energol oils are part of their legacy range and being
phased out, I had a nose around on their website, they seemed to suggest
Alpha SP as replacement

https://thelubricantoracle.castrol.com/range/alpha-sp/industrial/en-JP

But looks like you'll have to choose between going up, or down on the
viscosity ... maybe contact them?

In an "ordinary" industrial gearbox, one viscosity grade either way will
not matter. With a thicker oil, it will warm up a shade and drop the
viscosity. Similarly a thinner oil will run a shade cooler.

I wouldn't apply this principle to a modern military helicopter where
there may be narrower margins.

The important thing is to chose an EP lubricant so Alpha SP should be
fine. No need to stick to BP if you have another EP gear oil of the
correct viscosity.