When my 84 Westy had the same issue a number of years ago, I had to drive it back home with my hand on the shifter for 300 miles.  A rebuild was the only solution.

I tried changing the fluid before the drive home, found lots of metal shavings, and it helped me limp home.

Fortunately, it isn't too hard to drop the trans.

I use Redline, and my rebuild has purred for the last 15 years!
https://www.gowesty.com/product/lubricants/24358/red-line-gear-oil---1-qt-75w90-gl-5-

Jack 

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of WILLIAM WITZ
Sent: Wednesday, May 1, 2019 7:14 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Popes out of 4th gear

A few weeks ago on our way home from central Oregon coast about 55 miles from our home the van started to pop out of 4th gear on I-5.I checked fill as best I could. I did not see any signs of leakage but added one Qt. of Amsoil 75 w-140 Severe Gear Synthetic. We drove over to Pinnacles NP and drove home without any further problems from the transmission. I plan to drain the oil and refill it with the same weight Synthetic Oil hoping this will prolong the life. Any wise words from any one?
Bill Witz 84 Westy Tiico