Chaincase oil

... I use automatic transmission fluid on the basis that it's designed to work with ferodo-faced clutches like we use.

Has no anti-friction additives so ideal for primary chaincases (and forks and any small engines). It's a universal oil for turbocharged diesel engines, hydraulics and oil-immersed brakes. And I use it in my tractors and it's cheap.

I'm not a fan of any universal oil. Car oil, for example, isn't always suitable for wet clutches. As for tractor oil, my tractor has a dry clutch, so I can't vouch for it. If it's for oil immersed brakes, like Gordon's oil, a clutch should be fine too.

There will be many right and wrong oils, as you can see in the reactions. For me, the choice is dedicated modern motorcycle oil, as I know for sure that's suitable for wet clutches (which your dry clutch will be if you fill up the oil bath). Doesn't have to be full synthetic high-end stuff, just the most basic modern oil will be fine to keep your primary chain lubed.

I use Harley primary transmission oil, sorry about that but it works well, doesn't stick the plates and doesn't make it slip. I've tried everything else in over 200k plus miles and it seems to work for me. The good news is you can buy it from the inter web so you don't have to deal with Harley types 

Using ATF oil helps identify leaks as its red. .  Now I dont have any problem leakage i use whatever oil i have as long as its basic stuff and starts with the number 20. Chains are not fussy at all. Clutches can be.