Can 16A relay be used instead of 8A relay?
If a relay can open 16A circut with 12V, It can easly open the 8A. Am I right?

It's not that simple.
@ocrdu has pointed out some important highlights - especially about driving the relay.
But there are two important things that should be considered:

The load type the relay is switching (Motor, resistive, or capacitive)
Inrush current rating of the contacts.

Please note that a big common problem that can be seen on relays is contact sticking, which is mostly caused by the arcs on especially switch-on events (Think of it something like welding).
finder 40.52 is capable of switching a peak current of 15A. If the load draws a higher current even for a short time then the contacts may get sticked. So,

I have a relay with 12V and 8A, it was not working properly.

That may be the problem that you are currently facing.
Omron G2R-1E-12VDC has a max switching current rating of 16A. So, even the nominal current rating is higher than finder's (16Anom/16Amax against 8Anom/15Amax), there's still a risk of encountering a contact sticking problem. So,

Identify the load type
Identify the peak switching current requirement
Identify the max power