Oil for a Slant Six that's been sitting a long time

I've got this 65 Valiant that's been sitting for months since I've acquired it and years before that probably without running. When I first got it about 6 months ago I was able to get it started right up after a carb rebuild. The engine ran, didn't smoke, and didn't make any unusal noises. The oil in it is still black but not gritty or sludged. It's been sitting again outside for 6 months while I slowly get everything around it back in order. The whole engine bay was a mess when I got it and the interior even worse. I've got everything taken care of under the hood except a passenger side motor mount I can't find and a new Slant Six engine harness I can't find anywhere. But I would like to get that nasty old oil out while it's still sitting because it maybe a couple more months before it can be started. What kinda oil should I put back in? Just anything new? Should I put any kind of additive in to help protect when it starts up again after sitting for so long. It's a 66 170 motor, no idea how many miles on it, because it should have a 225 originally.