Gear grinds into 3rd after it has warmed up

[Aclosefriend]

Bought the car with 65k miles. Noticed slight grinding sometimes when engaging into reverse and really bad shuddering when letting the clutch out into 1st when the car was cold.

I changed the tranny oil with Royal Purple and the clutch shudder in 1st gear went away as did the grinding into reverse. For the last 10k miles or so(82k on it now) I have noticed 3rd gear grind really bad if I quickly shift from 2nd to 3rd. It never grinds if you put it in neutral for a second or two, let the RPMs fall and then ease it into 3rd. It doesn't grind when the car is driven cold, it only grinds after it has warmed up and you shift into 3rd really quick.

I am worried that either the syncro went or changing the tranny fluid dislodged some sludge and is causing problems. I have heard rumors of that happening if you have gone far too long without changing the fluid, but I feel like that shouldn't be the case if changing on schedule. Is it possible that the royal purple gets too thin when it warms up and is causing problems?