I've read the HJ FAQs on gear oil but one thing's unclear - the GL rating.

There's some Duckham's oil on ebay which is semi-synth and it says SAE75-90 and "GL1-5".

I can understand the multigrade viscosity but no the multi GL rating, my understanding is that there can't really be an oil which will suit all GL ratings.

Can anyone comment? Thanks in advance.