Turbo 350 help

[VT5LSS304]

Hey guys,
Have a turbo 350 behind a 355 in a vh. Between first to second shift it has started slipping just in between the 1st to 2nd gear change out of no where, doesn’t slip in the gears themselves, as in can do pulls in each gear fine and shifts into top gear normal. Box is manualised, meant to be built for 1200hp.
Any idea’s or experiences on what this may be??

 

[vc commodore]

Been a while since it has been serviced? Is the fluid bright red or starting to discolour?

Thinking along the lines of getting it serviced to see if that fixes it

 

[VT5LSS304]

It had an oil change not long ago. Don’t think it had the filter though. Might be worth a shot