Hi all my gearbox will only stay in 4th when its cold, As soon as the temp rises it drops back to third, the oil is dirty and more than likely has a flogged filter, would changing the filter and the oil help this problem???
It wouldn't hurt! Get a service kit for $30 from Repco or alike. That will get you filter and gasket and then replace oil and see what happens!
Be ready to pull it out for an over haul though. Sometimes changing the old clagged out oil is the only thing making still work. Nice new fresh oil sometimes causes it to then drop its bundle then.