when im in 3rd goin about 70kmh and i shift into 4th it kinda gives a suttle, krrght, not a grind but just has a little problem goin into gear, and thats only at high speed, not at 50kmh or so, is my synchro hooped or do i just need new tranny fluid, i have a ys1 btw, thanks guys

im thinking of getting new fluid anyway but what should i run to protect my syncros better? ive heard gm or penzoil synchromesh is good? would i run normal 80/90 tranny fluid or motor oil type? so penzoil synchromesh 5w30? or gm synchromesh i heard was better? do i fill the whole tranny with it or go half n half with something else, i havent done a tranny fluid change before so i dont know, once again thanks