Completely full of oil?  no problem with welding, other than the oil  contamination in the actual crack.   You'll likely see some pretty good  porosity  until  you burn off the oil,  so   make a tiny pass just to  seal the crack,  then   go back over and  lay into it with your filler pass.

I welded up a diesel tank after draining and  throwing  about  10 lbs of  dry ice  in there to displace  any oxygen.  I'm still here.