My personal experience is that I had no trouble using a 2011 11" MacBook Air i7 with 4GB of RAM running various versions of OS X up through El Capitan. Up until about 4 months ago, that machine was running perfectly fine for me until I re-aquired my daughter's 2013 rMBP i5 with 8GB of RAM (after we upgraded her to a 2015 i7 model with a bigger SSD that she needed for her Final Cut X use).I run both OS X El Capitan and Windows 7 on my 8 GB 2010 MacBook Pro.
Windows 7 seems to use quite a bit less resources. It will even run quite well with 3 GB of RAM, while El Capitan will not.
The nice thing is this means it will have longer usable life since OS X is becoming unbearable on it. Windows 10 ran beautifully with the exception that the forced updates ate up my mobile data and battery life while on the go sometimes... so I went back to 7. It's nice that the machine will be usable for some years to come.
Of course, a nice lightweight distribution of Linux uses by far the least resources.![]()
The 8GB in the new MBP feels super roomy for me, lol, for a laptop.