jkruss82

Lots of oils acceptable, see attached list of Engine and Transmission approved oils. 

Best not to use additives.

Mack approved oils.pdf

There may not be rebuild kits, but O-rings and seals should still be available if you can find the sizes or part numbers.  When you say shifter, do you mean the shift knob, the shift cover or an air valve on top of the trans?

Flywheel housings do have standard SAE bolt patterns that will match to all heavy duty transmissions.  You will want to pay attention to your clutch and flywheel selection because all OEM's have different amounts of clearance.  A Mack 9-spring clutch will probably not fit in any transmission you select.

Search part number 20QE3333 or 20QE3333A.   This is the part you need.  You should be able to find something online.

Some parts may be OEM NOS, but most are knockoff, re-engineered Alibaba crap. Its a shame that Mack gouges the price of parts the way they do in  aftermarket.

You might find that the shift cylinder needs new O-rings and seals.  Sounds strange that you wouldn't preselect the range switch, because the synchro needs about a full second to shift when working properly.

First thing to check is that your air pressure is good and that you don't have any leaks.  The range shift is air controlled so if there is any loss in pressure the shift may make a noise.

The 2070 model has a 0,60 overdrive gear so top speed will be about 75 at 2100 RPM depending on tire size.