Since I can't find any Castrol RS 10/60 would it be ok to top up with Mobil 1 15/50?

 

I don't think so...
Try Demontweeks or Catrolclassicoils.
JIM

 

From working in the oil filtration industry I can advise the following: If you really need to (off the dipsick) top up with whatever oil you can get hold of.

Reason: Any Oil is better than no oil.

If you can wait till you get some Castrol delivered then great, If not top up with whatever you can get your hands on.

I have done the same myself in the past with no problems.