Will the motherboard automatically lower the speed or is it incompatible?
You can try, but it's not guaranteed.
Intel Sandybridge 2000 series desktop processors natively support DDR3-1333, but the quality of the memory controller means that it will usually run all JEDEC standard profiles up to and including DDR3-2133 without much of a fuss. Running DDR3-1600 on this platform is technically overclocking but it will run flawlessly well over 95% of the time.
AMD's processors are a little more finicky. There have been reports of Bulldozer CPUs having a hard time running natively supported DDR3-1866.
Motherboard quality will also have an impact as power stability becomes increasingly important as operating speeds increase.
My recamend is fill all slots dont see capacity. see mhz in board and ram.Reason mhz is the key to achive speed. if board support 1600mhz u insert 1333mhz its waste because not get full speed of ram slot. if board support 1333mhz u insert 1600mhz its also waste of money (1333mhz ram is cheaper than 1600mhz)also u not utilise full speed of ram. (2x4Gb=8Gb 1600mhz=3200mhz =3.2ghz) vs (2x8Gb=16Gb 1333mhz=2666mhz=2.6ghz).
mobo support all rams automatically set default clock.
I am use 800mhz ram in 533mhz supported mobo without issues.
RAM does not run at the speed it is sold at, it is sold at the maximum speed it is guaranteed to work at. 1600mhz RAM will work at 1600mhz, or any speed less than that. RAM speed is set by your motherboard and processor combination.
In short, yes 1600mhz ram will work at 1333mhz just fine.