I need some advice here. I was doing an oil change on my 2011 P71 and realized I bought the wrong size filter. By the time I noticed, I’d already drained the old oil, and the parts stores had closed for the day. I really need to drive my car tomorrow morning, but I won’t be able to grab a new filter until the afternoon. So, can I go ahead and add new oil with the old filter still on?
Yeah, you’ll be fine to run it like that for a bit.
Just do an oil change again halfway through your usual interval. Should be good.
You might need to do the next oil change a little earlier, just in case the old filter didn’t catch everything.
Just change your oil again next time, no harm done.
You can swap the filter later without draining all the oil again. You’ll lose a little bit, but not a lot.
As long as your filter isn’t all clogged up or you didn’t wait too long to change the oil, it should be fine. Just swap the filter soon or shorten your oil change interval next time.
There are some interesting tricks you could try. One involves taking the oil fill cap off and having someone hold a shop vac over the opening to create suction. That way, you can swap the filter without losing oil.
@Aubrey
Pro tip right there! 
You can even find videos on this trick from people all over. Works great!
Filters are pretty cheap. Why not just swap it out when you can?
It’s okay to leave the old filter on for now.
If you already loosened the filter, tighten it back up, fill up with new oil until the dipstick reads full, and you’re good to go. Once you can, swap the old filter out and top the oil back off.
Trucktires gave the best answer on this one, closely followed by Diafuge!