Could you tell me which are the Memory Supported for Optiplex 990 Mini Tower?

Reading the manual it  seems only   DDR3 Speed 1333 MHz  therefore PC-10600, from Crucial seems to be compatible also with DDR3 Speed 1600 therefore PC3 - 12800?  

They are both compatible?

Where we can find complete information about RAM compatibility for this Opliplex and in general for other Dell Product?

Sometimes the manual are not so complete....