Just pulled a trans out of a 08 type s I was driving around for awhile and threw it into my cleaner bodied 08 type s shell, been breaking in the new clutch for about 350 miles now and fifth gear started randomly popping out while cruising under 3k rpm, no issues before this or with any other gear, have seen on other post to change to the GM fluid, I have the oem Honda in there now, but is there anything else I should be checking for?

 

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Try the ACDELCO 104014 friction modified fluid, see if it improves functionality. It can be found at rockauto for cheap.
Otherwise, the fifth gear & 5-6 synchronizer sleeve set may need to be replaced? It is still available as shown below.


There was a third gear grind / pop-out issue in a past TSB (attached) that called for the 3rd gear sync set to be replaced.
Maybe you'll get lucky with the 104014?

 

check the linkages and make sure all is right with it, maybe something got bent/pinched and is causing the issue.

 

My car was doing that, changed the fluid to ACdelco fluid and it stopped doing that. Could also be you have low fluid.

 

Got the fluids flushed n switched unfortunately still popping out of fifth, it didn’t pop out during the test drive after but the next morning it popped out on me, definitely not all the time, looking into the shift linkage next