Your first question really doesn't make sense. The operating system manages all of memory and will try to assign that memory in such a way as to provide as much performance as possible. Having a decent size page file helps the OS to make efficient use of memory even when the page file isn't used (don't ask, it's complicated).
One important thing to understand is that high memory use doesn't mean low memory availability. Most of the memory that's in use is also available.
The G510 can physically accommodate 16GB but Lenovo seems to say that it only supports 8GB. Several sites that sell add on memory offer 16GB kits for it, so it's near certain that 16GB works.