Conversation Between oilman and ScoobyFC

Hi
Ive a blobeye sti with dccd 6 speed box. Its developed a crunch 4th to 5th and im looking to replace the oil before condemning the box. Whats my best option? Feeling confused as ive seen pros/cons for redline stuff and castrol syntrax stuff.
Also I'd like to do diff to if you could advise
Thanks