1st, Co grats on your crossfire.

how many miles on it ?

in some transmissions use 75w90 but DO NOT do this in a synchro trans, it will eat up the brass snycros.

I use royal purple in mine, change every 50k miles.
and when you drain the oil you can see how much brass is in it to tell how previous owners drove it.

I know the clunk you are talking about.
I bought mine with 50k miles and at 170,591 atm, always done it when cold, a better oil will help.

but pay attention to the clicks and sometimes you will feel a double click going into 1st gear, and will help so you don't get the 1st gear pop out.