You're right. I did a quick look in the operators manuals and focused in on the utility tractors only early this morning and headed out the door. Sorry about thatů.
The manuals for the 300 Roundnose and 300B stipulate the following:
General Purpose: 5 Gal
Utility: 13 Qrt
There seems there are a number of capacities over the full production run of the Rock Island tractors.
If I remember correctly 5 gal was not enough to reach the full line on the dipstick when I replaced the gear lube in my 312, but I don't know if it is any different than the 311.
The early 430/530 general purpose call for 6 Gal. The later 430 calls for 26 Qrt.
The 351, 500B/600B and 630 all call for 11 Gal.