I rebuilt my '20 SR/S frontend a month or two ago. I used RaceTech 5w fork oil. There are definitely differences in actual oil viscosity at different temperatures between each brand (One brand's 5w is another 10w - see image), so the important thing is to pick an oil that feels good for your setup and stick with it.
In other bikes I've owned with Showa BP-SFF forks, the spec oil was Showa SS-47, which I understand is a replacement for SS-8. If you're having a hard time finding Showa oil for a reasonable price, you can try looking for Honda or Kawasaki-branded SS-8/SS-47 oil, which should be similar. 10w oils from other brands like Bel-Ray, Maxima, etc should be close as well and much cheaper.
I forget what I set the fork oil level too. I just matched whatever was in there when I took them apart. Other bikes I've had were spec'd around 90mm.
For steering head grease, I just used a Bel-Ray waterproof grease. Any good automotive waterproof bearing grease should be fine, just make sure to properly clean and re-pack them.
My head bearings were tapered bearings, where torque values are basically useless. I assume yours are too. When setting tapered bearings, it's all by feel - you want them tight enough so when you try to rock the forks you don't feel any movement, but no tighter. It's better to be tighter than looser. There are good tutorials on YouTube for what to do. Just make sure you check them again after a hundred miles as they like to loosen up after being set.
For the upper triple lock nut, I just tightened by feel, since I couldn't find a torque value either.