The Oil Sight Glass lets you check your oil level without having to take out the oil level plug to see if you have enough oil in your engine.
They’ve been around before, but for a while, they always seemed hard to get a hold of, but Oiltek are selling the Chaincase Oil Sight Glass.
The window on this one is made of glass, but they do also come with a plastic window. It also has a piece of alloy behind the glass to make viewing of the oil level easier so you’re not struggling to see the level in the darkness of the engine.
A point to add. You do have to wait for the oil to settle after running the engine for obvious reasons.
You can order them here from Oiltek