With a machine of this vintage a lack of oil may not be the problem.
If this machine has a removable top, remove it. Remove the face plate too.
If it can be turned it doesn't seem to me it's locked from lack of oil.
Check everywhere for thread wound up in the mechanism.
>Especially under the face plate in the thread take up lever parts
>Over, under, around and through the bobbin area, pay careful attention to under the hook
>Pull the hand wheel and check out around it
I've had several machines come to me with thread wrapped around parts. In two cases the machine was totally immobile because of it.
Joe