Removing the lowers quite a lot of dirty grey oil came from the leg with the air valve on. It looked different to the oil on the other leg, which looked like the fork oil I added last time I serviced the fork. Should I be concerned?

If you’ve had them a while the main seals probably need changing.

I find the left seal (brake side) wears faster than the other. Seems very odd but I suspect the left leg deals with more torsional force than the right because of the brake location

+1 to the OPs situation
+1 to the cause and fix.

Cheers guys. Are there any guides to changing the main seals online? I was planning a lower service replacing the dust seals anyway.

Check out x fusion own website have a range of videos for lower service and wiper seal replacement.