will adding atf in a 5spd loosen....

just wondering if i add some ATF in my 5spd, will it thin my gear oil a bit...i put some thick gear oil in my car and im too lazy to drain the gear oil and add more oil again plus i dont have a good tool to remove the tranny drain plug. ive been having problems switching gears in the cold mornings because the thick gear oil. thanks for the information in advance.