[RWT]

I can't speak to your track/driving temps but I have owned at least one manual transmission vehicle at all times for about the last 45 years. I can tell you there is no advantage to overfilling the transmission. It will not help with temps and can in fact help create additional heat. Overfilled transmission fluid will foam (aerate) from the churning (instead of splashing) in doing so it doesn't flow well. When it doesn't flow well, it doesn't lubricate well. When it doesn't lubricate well the parts heat up and because the fluid is foamy it doesn't carry away heat well. This is an oversimplification but I'm sure you get the idea.

As for the Redline d4, some lubricants just don't work well in some applications. I would go back to the Amsoil (I think it has more unicorn urine in it then the Redline anyway) or even try the OEM stuff and see how that works out.

The previous owner could have used an improper fluid and damaged the bronze sintered components (Synchros). GM hasn't officially licensed any DEX III since 2011 and it has been superseded in almost all automatic transmission applications. But it's still used in manuals because it doesn't damage brass or bronze components.

Hopefully some of the manual SS guys will chime in about your temps (I'm clueless).

No matter what you choose, just fill as recommended by the manufacturer and if it's still overheating and/or not shifting properly it may actually have a mechanical issue.

Please keep us posted as to your outcome as it may help someone else.

JMHO
No expertise implied or expressed