My 86' GL has this same problem, when your going like 5 miles an hour and you still cant get it into first, and shifting into 3rd form 2nd or 4th is hard and sometimes grinds. First of all, use a broad range weight tranny gear oil, castrol has a good 75w-140 for like 8-10 bucks, secondly, do you know how to double clutch? when coming to a stop and want 1st gear, put tranny in neutral, and rev engine(based on speed that you want to downshift) and then push in clutch and put into gear, what this does is get the first gear gears going near the same speed so the worn synchros have an easier task of mating the gears together.
~Josh~