Early Slant 6 Rebuild

Are you certain it’s motor oil? Sudden huge leaks don’t happen that often to the main seals. Transmission seals, especially in manuals will let go on a guy...
It’s definitely motor oil. Haven’t had to put in any trans fluid in a year, it’s still got plenty. But ran low on motor oil 2 weeks after an oil change. Oil pan is tight and back oil drain plug is tight