Motor oil is just fine. Thicker is good though.

I've started using Chain-L in the summer (on the road bike), Pedro Synlube in the winter to help put up with salt and grime.

Chain-L is, as far as I can tell, just thick motor oil, like maybe 60 weight. Gear lube sort of range.

I've given up on everything that's thin. I tried quite a few thin bike chain specific lubes and none of them worked at all for me, including Boeshield T9