Bezzer,
I just found this thread. Several months ago I had to replace the top vertical shaft gear on two Singer 413s. Same exact gear as the Singer 457 that started this thread. I was told to set the gear so there is no play in the shaft after the new gear is installed. The thing is, if you get the gear too tight on the shaft, in other words zero end play, you will or can run into a tight feeling machine like you describe. I followed the instructions I was given and got the machine so tight the motor could hardly turn it. I realized there has to be some free play in that shaft or it will bind.
I don't know that this is your machines problem, but it's something to consider after you have oiled everything. The only way to loosen it, IF that is the problem is to pull the plastic side plate off, stick in a bit or screw driver and loosen the set screws without rotating the machine. If it's too tight it will loosen up after the screws are loosened up and retightened. At least that's what mind did.
Joe