Vintage Minor Register

I have an English friend in California who runs a restoration workshop for our sort of cars. This morning, he sent me this:

"I have a friend who has a 1930 MG M-type. His problem is that it changes gear OK when the gearbox oil is cold but as soon as he has driven a few miles, it becomes impossible to change. The gears will not engage, despite double-declutching, etc. Have you have ever encountered the problem or heard of a cause (and fix) for it?"

Any ideas guys?

Interesting!
It can´t hardly be the oil itself but more likely the effect higher temperature has on the mechanical parts. But you need more information. For example is it possible to change gear when standing still/engine not running, standing still/engine idling, can you hear (and/or feel) the "teeth" interacting when trying to change gear and so on.
I don´t know about M Type gearbox but I'm rather familiar with gearboxes in general and a first guess is that the problem is connected to worn fork/fork lever. But, I believe that more information is necessary...
/Thomas

Thanks Thomas, most grateful. I will relay this and post the response here.