I am backing up my MacBook Air that runs OS X El Capitan 10.11.6 to an external Hard Drive using Time Machine. I have another older MacBook that runs Mac OS X 10.6.8. If I wipe the hard drive of the older macbook can I restore it using my the Time Machine that was created using OS X El Capitan? Any issues that I should be aware of?
Each mac can only run versions of macos that apple allows.
You need to look up your older machine in a database like http://mactracker.ca/ to find out if it supports el capitan.
There is a five release difference between the two macOS. The new OS will use much more ram and run slow on an old machine.