MGB 1966 MGB hard shifting into 3rd

I haven't driven my '66 MGB much since it was rebuilt about a decade and a half ago. (I won't go into it.) At that time, it received a rebuilt transmission and overdrive from Moss Motors. I've recently started driving it, and I've noticed it is hard to shift into 3rd. It's more difficult shifting up, but it's also hard shifting down. There is no grinding or anything. It just seems to be hard to find the 'slot' and it takes a bit of force. The other gears are fine. I mentioned the overdrive for completeness. It doesn't seem to have anything to do with the shifting issue.

Since it's a 'new' transmission, does it just need to be driven to 'loosen it up'? Or might there be a mechanical problem?

Have you tried double clutching it both upshifting and downshifting? If so, any difference in the effort to get into third?

The three-synchro uses motor oil as lubricant, have you made sure of the fluid type and checked the level?

IMHO, putting more running time on it would be the first thing. 500+ miles, at least. If there's no change after that, only thing left would be to remove the unit and look for a mechanical problem.