Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

I often use penetrating oil or if it there is caked on hard water/calcium I put some CLR around the glass/fitting.  Though CLR can/will eat away nickel plating, so I've learned to be careful.  I can't say I ever had a engine with that setup like in your photo but once I get the slight glass off the engine these are my steps.

Soak sight glass in CLR (calcium cleaner).
Use a tooth pick to poke paper towel in (soaked in CLR) and work paper towel back & forth.
Some sight glasses you can't get a tooth pick in, so you have to live with paper towel - which is waaay fun ;-)
Rise sight glass with water
Use tooth pick to poke paper towel with glass cleaner this time and work paper towel back & forth.
Repeat until I am happy as I can be with the sight glass.

Hard water calcium can be a pain and can take time to get cleaned off.  Just avoid anything harder than a tooth pick, since you don't want to scratch/damage the glass.