The pump on the hydro's has some very tight tolerances... any kind of grit or foreign substance can be detrimental to the pump.. that is why they have a filter... The gear drives need clean oil also but the tolerances aren't nearly as tight so they can stand dirty or incorrect fluid without as many concerns..
I wouldn't worry for one second about that 1806.. just change the fluid and be done with it..
Good luck with it! Keep us posted! 
__________________
Jay
40 years of Using and playing with IH Cub Cadets!
Proud owner of the following:
Cub Farmall, Super A Farmall, Original, (2)70's, 72, 100, 102, 123, 105, 125, 127, 108, 128, 1450, (3)782's, Yellow 982, 1782, "Sam's" 2182, M Farmall and a #7 trailer
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