[Farmall] 340U lift problem "fixed" - what should I do next?

[br549 at hemc.net]

I have a IT brand 340 service manual as well as the reprint of the 340 
owners manual. Using them, I checked the transmission oil level. 
When I removed the oil level plug, no fluid came out. Even after 
adding TWO gallons of trans / hyd oil none had reached the "full" plug. 
My lift does work correctly now but from the excellent responses I got 
here, I am concerned about the filter and possibility of a leak in the 
system. 

Do you recommend replacing the filter and topping off then watching 
the fluid level? Should I drain all fluid and replace? Is there any other 
recommended or protective / preventive maintenance that should be 
done on the system?

Thanks for all the help,

b