First, we do not recommend usage of Synthetic type oil but the usage will not harm the unit either. The only reason we don't recommend synthetic is because it creates a film on clutch friction surfaces and our plates don't lock and slip smoothly. Use of synthetic oil normally create more chattering noise because the lock to slip and back to lock operation becomes more abrupt using synthetic. The oil may quite down the unit initially but once the oil viscocity goes down, the noise will noticeably get worse.

This information is from many of our road racing clients who run NASA and SCCA Touring Cars. "Redline oil chews up Honda synchros." Maybe half of the guys running the Nasa US Touring Car series uses our oil and the other half use other brand oil (Redline, Royal Purple, Motul etc...).

Bottom line is that the choice is really up to the users. Synthetic oil will not harm the unit. We just don't recommend the usage. We have a user in the Bay area who buys our oil every other month for his road racing Civic and he doesn't even have our lsd unit. He just likes the shift feel with our oil so he keeps buying them. So the choice is up to you.
If you like the transmission feel using the Redline, you should keep using it.
We only use our own oil or the Torco Racing Oil from www.torcousa.com in our company race cars and also on the Time Attack Civic currently leading point in the Redline Time Attack Unlimited Front Drive Class (Driven by Tage Evanson).