I have a 05 T5 Transporter 2.0l with a 5 speed gearbox 105000km. It has got a problem with the gearbox.

After driving for about 20 minutes from cold, it develops a crunch as it goes into 3rd gear form 2nd or 4th, by the sound or I would think it is a syncro. However if it is a syncro why is it that it only does it when it gets warm?

My local mechanic put some new oil in it & it now takes 20 minutes instead of 10, is there some special oil I can use? has anybody else had the same problem?

I am trying to avoid getting the box overhauled if it is not required.