05 MT became very difficult to shift into first and second almost immediately after having MT gear oil changed. I went back to mechanic and he showed me that the fluid he used was 75W85 and met Nissan specs. However, the change was so obvious and shifting so hard that I bought four quarts of Nissan fluid and he changed it again for free. Problem was completely resolved as I left the mechanic. I do not know what subtle differences or additives the OEM fluid has but I will not be using anything else if I ever need to change it again.