[Synchro909]

Don't use STP. It foams.
Never take your Model A to a dealer for anything. They know nothing about them. A couple of years ago, I bought a 1930 CCPU out of Arizona. The seller had taken it to the local Ford dealer for service and other works. What a disaster!. They had relined the brakes and they were so far out of adjustment that they were dangerous. They had replaced the ring gear (on the flywheel) but the put it on backwards. Clueless. They greased some of the fittings but not all and they put the mileage for the next oil change 3,000 mile sdon the road. How stupid can you get - the car has no oil filter!
I could go on and on about their incompetence.
Back to the OP.
If the gearbox has been filled with a low viscosity oil, replace it with the proper oil and I think you'll find all will be well but after you press the clutch pedal down, pause for a moment before engaging 1st gear.