The Schaubling 13 oil level sight glass should be replaced because the original one was chipped off after pulling it out. Which materials and how to mark it?

This is the original oil level sight glass of the Schaublin 13 (left). The window was dirty as usual by years of oil contact. It needs to be cleaned. When I pulled it by threading a couple of screws it became chipped off.

Would be nice to make a new one. The original one was made of methacrylate, so I would replace it by a new one of the same material. Just turn it to size from a sheet of methacrylate. The reflector is also made of (white) methacrylate, threaded and glued with cyanoacrylate to a setscrew of a suitable length.

Level lines where engraved by using a thin metal saw exactly at the same distance as the original ones. Now engraving should be filled with color. Which material can be used that resists the oil and also the fuels that can be used to clean time to time?

I prepared a test with several engraved lines and filled them with distinct products.

These are the products tested:

  1. Edding 780

  2. Markal Paintstick+ Lacquer Fill-In, a marker specific for filling engravings.

  3. Edding 780 covered with liquid Tipp-Ex, checking if tipp-ex is useful protecting the Edding color.

  4. Nail polish

  5. Two components Polyurethane coat

The test plate was plunged into machine oil for a week and then into a mix of oil and gasoline fuels during a second week. The result was as follows:

  1. Edding 780 was dissolved by machine oil in some hours
  2. Markal stick become more creamy but do not loosed its color nor flooded away
  3. Tipp-ex was dissolved in some days by machine oil
  4. Nail polish had no significant change during the two weeks
  5. PU had no significant change during the two weeks

Then, nail polish and 2k PU seems to be the best marking products according to oil and fuel resistance. I choosed red PU to fill the max and min lines engraved in the new oil level sight.

The engraved lines were filled using a syringe and a small needle as shown in the next video:

After this, the dryed paint is carefully scraped where needed and overall is polished by usen jewellers rouge and water:

This is the result:

Finally, I mount the sight glass in the milling machine. I used a small amount of liquid gasket to guarantee its oil tightness.