I had a very bad exhaust leak
&
I had the wrong gear oil

the manual says to change into 5th gear at 45

It was not grinding when I changed into 5th at much higher speeds sometimes.
I got the exhaust fixed and the car started performing MUCH. Better! No more stauling while trying to get going in 1st as much.

Gear oil, Lucas synthetic was all I could find, now the car still grinds in 5th until the car heats up then it goes away...

I can see this being a problem to high flow exhaust enthusiasts unless they have a open & close valve once in 5th gear.

Hmm, are you saying your exhaust leaks are related to grinding gears in the transmission? That doesn't sound correct to me. I think I am misunderstanding what you write. What are you telling or asking?