Yes, there's a transmission filter.
a
No you cannot change a 13 to a 18 speed, however you can change a 18 speed to a 13 speed just by changing the shift knob.
50W if synthetic, 80W is mineral.
Either a proprietary Mack 18 speed or an Eaton-Fuller 18 speed. Various models of the Eaton-Fuller were available, from 1600 torque to 2100 torque.
No. Unless you have a "C" transmissions (e.g., RTOC, RTLOC, etc.), it's not convertible. A 10C transmission doesn't convert to a 13 speed - a 9C transmission does. A 10C converts to a 15 speed, which remains essentially a 10 speed with five deep reduction gears.
I've never heard of an Eaton Fuller 12 speed, nor am I able to find any references to one via Google. I've driven Eaton Fuller 8LL, 9 speed, 10 speed, 13 speed, and 18 speed transmissions - could you be thinking of one of these? IIRC, Volvo marketed a proprietary 12 speed transmission, and Meritor offered a 12 speed automatic, but not Eaton Fuller.
Depends on the final drive ratio.
The 6 speed manual does not have a filter.
No, 10 speed does not convert over. You would need to get a truck with either a 13 speed, or a 9C transmission (which is convertible).
Keep rpm between 1400 to 1600
50W synthetic, or 70W mineral oil.
yes .. Change filter, then try output and input transmission speed sensors.