The 516 is basically a 514 (same arm, same automatics), with a far more advanced belt drive. In the 514, the speed is not actively controlled and they tend to go off speed over time. In the 516, there is an oscillator that controls the speed. This gives you extra speed control and a strobe light.
I have experienced that long term mainenance wise, you can do 2 things to a 516. If the speed fluctuates, first clean the three speed pots (the main control pot and two pots on the board) with contacta or similar. If the speed then still fluctuates, check whether the motor runs smoothly. The motor tends to accumulate dust and become clogged over time. Disassembling the motor and a good clean solves that. Another general thing you can do is to recap the power board, as these caps are now 35-40 old and will fail sooner or later potentially destroying your unobtainium speed oscillator.