Don't worry - it'll be just fine. Engine oil is quite benign to the sort of rubber used in power steering so won't eat the seals, and it will mix just fine with the green PSF that should be in there. It'll end up a bit thicker than it should be but mixed at the ratio you suggest it won't do any harm. Flush it out with PSF and it will be just fine
The system holds about a litre so get at least two. Put a full litre through it and then full up with a second litre.