Here is a thread from my Pantera brothers which gets debated about every 2 years. I use Castrol 80W90 which is what Roy Butfoy told me 34 years ago and never have had a problem. NON synthetic. They drip as they are. Synthetic will cause a lot of leaks.
Somewhere between 3 to 4 QUARTS. That post you made about the dipstick from a zip tie is one of Lloyd Butfoy's pieces of info.
Here is a list of Conventional (mineral based)::
Castrol 80W90 (Note it is now GL-5) which is fine
PennGrade Classic GL-4, 80W-90
Driven Racing Oils, GL-4, 80W-90
Richmond Gear, GL-4, 80W-90
VP Racing Classic, GL-4, 80W-90
Liqui Moly Gear Oil, GL-4, 85W-90