I use Silkolene fully syn (Pro S) 10/50 IMO you should have a 50 weight, dosen't cause as much drag when cold as well like a 15 weight would.

The actual reccomendation for these is 5/30 but IMO that is to thin

I always fill mine below the max line.

The actual capacity is 3.0 litres without filter and 3.4 including filter.

Martin