Sorry you're having an oil spill ;-(
but your in good hands with Steve.
Sure you can advance the timer while the machine is on--another wonder and glory of vintage machines. Slowly and gently, of course. With the adjusted water pressure, the fills are right on time.
Interesting about timer increments: On the WO's they're half minute increments, which is kinda fun because it click click clicks away, zipping right through the cycle. Then in 1956 or so, Frigidaire switched to 40 second intervals between clicks, then in 1962, they went to one minute stretches. Not sure about exact dates, estimating from my own machines.
The 56 I have overfilled because it has a new I minute interval timer motor on a 40 second interval dial. Nice because you get a longer wash, 15 minutes, and rinse, 4 minutes and DRY, 7 minutes. And boy-oh-boy are they ever dry. With the adjusted water pressure, the fills are right on time, and the longer overflows compensate for the reduced pressure.
Again, good luck friend and be glad to have Steve. He's very precise, exact, and patient, and he loves to teach Frigidaire.