The gearbox sounds like a tractor

In 1st, 2nd, 3rd it sounds really rough but out of gear it sounds fine so I think the tranny in making a bit of noise. Went to halfrauds earlier to pick up some tranny fluid but the book was about as much use a chocholate tea pot

What do you guys put in your boxes? Should I just put some slick 50 in? Some guidance would be greatly received
