Tank filler drain

I have been going throught my new to me GRiSO, 2 days ago i did the swing arm/shock bearings, all ok, there was very little grease on them, but they are swimming in the stuff now, yesterday i decided to check nothing trapped under tank.
i noticed that the drain wasnt working in the tank filler, this is the one that runs to the left side rear of tank, i poked through with wire & got it draining, however to my mind its not draining fast enough, any tips for this please, i was going to unscrew the nipple where the plastic drain hose joins at the back of the tank, but decided to slow down & check first.
Further question, is Castrol Syntrax 75w140 ok in gearbox & Diff, its what ive put in mine recently, i used it on my BMs in the past & its been fine, the gearbox oil was like new, diff was murky when i drained them, bike has 3500 miles.
Thanks
Stevie

I had this with mine unscrewed the nipple made sure all was clear reassembled and all is good.