Type of oil for 5-speed manual gearbox ? HDJ 100 2000 model

[Cruisera100]

Time to change oil in the gearbox. The 2nd gear (syncro ) has allways been a little - how should I say - hard to get in gear when it's cold. I Guess it's a common problem with these. I read somewhere that, some types of oil make a big differance on these gearboxes. Castrol vmx-m ??? If anyone has any recomendations , I'd like to here about it - and how much is needed . Thanks !