ford 8000 tranny noises

[markct]

i have a ford 8000,the tranny makes a groaning sorta noise in 3rd and 4th gears, and also 7th and 8th since they are the same set of gears, it has done this ever since i got the tractor last year, also 3rd gear can be a bit hard to "find", like the lever has to be in exactly the right place to engage. i was just wondering if it could be as simple as an adjustment problem with the linkage maybe, or do i have a bad bearing or gear more likely? i guess it never realy bothered me much before but now that i am haying i am using those two gears alot and its got me wondering what could be causing it to make such a sound. another thing i was wondering is what about running a heavier oil in the tranny also, the tranny is seperate from the rearend and hydraulics, but it calls for the same 134 universal hyd oil, would gear oil be ok maybe. when i first had the tractor i changed the oil in the tranny and it looked sorta brown, almost like it was rusty possibly, but i dont know what kinda oil it had in it then. this is the early style splash lubricated tranny, not the later pressure lube type.