You can do it at pretty much any RPMs, so long as your timing is right. I can hit gears upshifting at 1300 or 1900 RPMs with no problem. What the ideal shift range is will depend on the engine. As a general rule of thumb, most people say 1400 - 1600 RPMs... personally, I like to shift a bit higher, and drop back down into the powerband when I go into the next gear, so that the engine isn't as prone to bogging down.
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.
Installation of countershafts
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.
Grey, both on an Eaton-Fuller and a Mack 18 speed.
It's the same as the shift pattern for the Eaton Fuller nine speed transmissions, albeit with additional gears on the low side. For the sake of not wanting to type out a long winded explanation, there is a related link posted - 'Eaton Fuller 18 speed shifting diagram'. Click on that link to see a visual representation of the shift pattern.
Keep rpm between 1400 to 1600
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).
9 speed doesn't have a gear splitter, just the range selector.
Fuller Transmission shift diagrams.There are several places on or offline where you can obtain and Eaton fuller 15 speed gear shifting diagram. It depends what method you prefer. such as from your Truck dealer or a Truck Transmission Re-builder or someone who offers fuller transmission parts and service. For more information of sources online, see the Related Links section further down this page.
50W if you're going synthetic (recommended), 80W if you're going mineral.
No you can not, it is a different transmission. The 18 speed has a splitter for the lower range as well as high range.
Google images. If you want the sticker, any truck dealership will carry it.