I recently cleaned and lubed my chain, havent had a chance to ride it yet, but if I leave the chain alone for a day or 2, it seems to seize up. It takes a lot of effort to backpedal it free. After that it seems fine. Does this mean the oil is not able to reach the inner workings? Or is it a reflection of bad lube? I use https://www.fasttech.com/product/1392817 (I am not asking if the lube is bad, but rather if simply bad lube might cause this. Havent had issue on other bikes with singer oil), or are there any other things that might be the issue?
My chain did something similar but less severe, I was cleaning it and adding dry lube from a posh brand, then keeping it and riding it indoors. The chain got gummed up and harder to turn.
What I needed to do was to clean it more thoroughly. I might have been breaking down some of the oil every wash but it must have been leaving a cumulative sticky residue, 1, adding friction and 2, preventing new lube reaching every inner surface. What I was doing was effective for a different lube but not the one I had switched to.
Try giving a more thorough wash (degreaser first) and use a sponge on the chain rollers to carefully work the dirt out of the links. Then rinse thoroughly. Dry the chain with a towel and allow it to dry fully before adding new lube to every link and turning the chain backwards to work it in. Wipe away excess lube.
I you think any dirt or residue remains when you’ve rinsed the chain, repeat the cleaning.
I don’t think ‘bad’ lube would be the main cause, because you should be able to clean away any bike lube with the right cleaner.