bendaw

I've a 06 cat D Forester 2.5XT, only 7000 miles
5th jumps out with great force on gentle acceleration after 20 miles at 70mph, then every few seconds until I let it cool down.
Drives at 50-60 all day (or at speed in 4th) with no problems.
Selector engages easy and solid.
Accident damage pushed engine and sub-frame up and back 2 to 3 inches.
Still has original oil in diffs and gearbox
Can't afford rebuild at present so hoping for easy fix (Please, God)
Could a change to synthetic high temp oil be answer?
Thanks