Hi! We meet again!
I can't remember the rule-of-thumb on oil pressure; it was posted here (somewhere) not too long ago so maybe one of the engine tech guys will chime in here (I think it was 10 pounds per 500 rpm). Mine runs about 50 to 60 psi at cruise speed and maybe 25 or so at idle (as I recall at the moment anyway!).
Again, we've got some pretty savvy mechanics here at the can go in depth on the transmissions but from a user standpoint...
The first letter designates the general use; that is the "A" is (relatively) light duty, an "M" is medium duty, and an "H" is heavy duty. I believe the "T" is just transmission or perhaps the designation for all automatics. The first number is the series...500 series, 600 series, 700 series, etc and goes along with the A, M and T. So you'll see AT5XX, MT6XX, HT7XX and such. The second number is the number of gears, and the last number is...
...I can't remember (I think it's the design level number or indicates a particluar feature of the transmission). So the AT545 is a light duty automatic with 4 speeds; the MT643 is a medium duty automatic with 4 speeds, the HT750 a heavy duty automatic with 5 speeds.
The AT545 does not have lockup on the torque converter, meaning that when you're cruising on a fairly flat road at a steady rpm the torque converter does not lock and therefor there's always some slip (and therefor lower mileage). The MT643 does have lockup (feels like you're shifting into 5th gear).
The transmissions are rated for use with regard to horsepower, GVWR (gross vehicle weight rating), and engine torque. Generally school districts would go with the AT545 unless one (or more) of the numbers above got exceeded. Since school buses really normally fall into the "light duty" category with regard to vehicle weight and since they're typically used in or near town a lot of buses got relatively small engines and the AT545 tranny. But sometimes an engine choice was made that required the use of the MT643 transmission; that's considered a "good" thing by a lot of folks since even in the most severe school bus application that transmission is considered under-worked.
There should be a placard on the engine that lists the max rpm; if the governor is set for that rpm there isn't much you can change with regard to that. If you can get full rpm (that is, the governor hasn't been turned down) then the speed is being limited by the rear-end gearing. The Allison AT545 and MT643 have a 1:1 final drive ratio so that doesn't affect the road speed (in the sense you're asking about). There is usually a tag on the rear end that lists the ratio of the unit. There are lots of (free) online speed/rpm/rear end ratio calculators; once you know the stats you can figure out the rest. For instance...if you know what tires are on the bus (size-wise) you can look up the diameter for that tire (Google search)...or just measure it on the bus from the center of the hub to the ground (times two of course). If you then know that the bus is hitting top rpm you can input the rpm, the tire diameter and road speed and get the rear end ratio. If you know the rear end ratio, tire diameter, and rpm you can get road speed. And so on and so forth. In some cases you can change the tires for those with a larger diameter and get a higher speed but most of the time you'll need to change the rear end ratio to get the speed you want.