Quinn, thanks, you're right. Forgot about that as I was not thinking about running oil level
Kevin,
The straight through sight would be worthless with the engine running, but maybe if it had an "anti slosh" cap in the inside (opening covered but a small hole top and bottom) it might still work.
3" nylon sight tubes are about $18. They have a 1/2" NPT fitting top and bottom. They might do better for running oil level.
A variant of that is something homebuilt like the Murphy L100 oil level switch gauge; it's a small externally mounted box with glass/lexan face plumbed top and bottom to the sump. There's has a float with contact in it. It should work find running or not.
I've got a neat little polypropylene float/swith unit from Grainger for $13. I'm thinking about how to best use it, and whether to use a sight plug for visual oil level inspection or make something like the Murphy unit. The little float switch is so small that that the unit would fit into a 1 1/2" pipe with the float about in the middle of the pipe. Its only 1/2" in diameter.
Best Wishes,
Bruce