Park vehicle on level surface. Drain oil and change filter. Add 4.5 quarts, start engine and run for 45 seconds. Shut down and wait 45 minutes. Check oil level and add as necessary. You now know the capacity. Do not overfill.
I have a 1969 Jeepster Commando with a Buick 225 V6, the forerunner to the legendary Buick 231 V6 still used today, with a Duetsch D431 oil filter. It will probably fit any Buick V6 ever made.
On the rear or side of the oil pan
4.5 quarts for the V6.
The Buick, Chevy or Ford 3.8 V6, it makes a difference.
5 QTS
That is an indication that head gasket is leaking. yes!
If it has a four cyl engine it takes 4 qts of oil, if its a v6 it takes 4.5 qts of oil.
Maybe oil passage leak by gasket in timing cover
Any major brand 10w30 high detergent oil.
Around 40 psi when at at operating temp.
Look at the rear of engine right side
how to install dist. on a buick 225 v6