Hey donutslayer,
So we don't recommend synthetic gear oil because during the early stages of a synchros life they need some friction in order to "seat" or break-in. You didn't cause any damage but rather it will A) take a long time for the synchros to break in, or B) the synchros will not break in at all.
So no need to worry, simply drain out that nice Redline
and save it, then replace it with standard mineral-based 75-90w GL3 or better gear oil and run that for the first 1,000-1,500 or so miles. Then replace that oil with the Redline you've got in there now and enjoy
BigMike
