BPI did make a dedicated real repenisher. I know, I still have some.
Ask BPI if they will make a run of pure real repenisher if there is sufficient demand. The use of real replinsher makes 777 even cheaper to use. You add only 3/4 ounce per 80 Sq. In. ( 80 Sq. In = 4 sheets of 4x5, 1 roll of 120, 1 36 exp. 35mm film). Actual replenisher is much more desirable than using fresh developer. You can tell them apart by the color of the labels. One is red the other blue.
777 is definitely NOT!!! sensitive to air. Because of the PPD it does normally discolor, but this has NO effect on its working ability.
A simple solution is to use 3 quart bottles and 2 pint bottles in place of a gallon bottle which will eventually fill with air. Even when you are down to the final dregs of the gallon, you simply use ever smaller bottles. You use it so slowly, even this is not any hassle at all. An emmently simple cure to what seems to be vexing imponderable.
"I only replenish every 7 or 8 sheets (or rolls)"
That statement only equates if your sheets are 8x10. Either way, you are replenishing too little especially if you are using 1 gallon or less of developer.
Fred