There are two petcocks on a Model T engine.
If oil doesn’t come out of the lower petcock when opened, there isn’t enough oil to operate the engine.
If oil comes out of the top petcock when opened, there is too much oil to operate the engine.
More importantly, I learned that both petcocks are unreachable and unseeable from where you add oil to the engine, and as an added bonus, I learned that any extra oil dribbles out of the upper petcock very slooooowly.
You could get sight glasses to install in the lower hole from period aftermarket suppliers. You still have to get under the car to see them. One risk is that if the glass breaks, you have an oil leak.
I was lucky enough to find a petcock with male pipe threads on both ends, so I made my own sight glass setup. I only open the petcock when I want to check the oil level, otherwise it stays closed. 
From my experience those gadgets are a constant oil leak and in my case I had to remove them.
A dipstick, although not original, is much more convenient. The 1st picture is of the one purchased from one of the parts suppliers on my 26. The other is one I made for the 19. I just dont feel like crawling under the car.
Mine has been working fine for several years. I am told that the ideal oil level is midway between the petcocks. Using the sight glass tells me that the T only requires 3 quarts of oil, not the 4 I was told. With 4, oil will drip out the top petcock. 3 is right in the middle.
The store bought dipstick works well I’m sure. But the homemade one interests me more. Would you mind sharing some specifics as to how you fabricated it and an estimate of its cost?
Thanks kindly in advance,
Tom in Taylor Mill, KY
I had to do an oil change on the 26 so I took it off and copied it. Neighbor gave me a dipstick that I modified to match the other one, I used a piece of 3/8 copper tube that I had and curved and cut it to match. Copied the firewall bracket and got lucky that I had a rubber grommet that fit the tube. The only thing I had to buy was the 1/8 npt to 3/8 compression fitting in an elbow. Don’t remember what I paid but it’s not an expensive fitting. It was fairly easy because of having something to copy, but I’m sure that with some patience and time one could figure it out. The fitting is installed in the lower petcock position.
