Octane fuel to use for a 92 Passat CL

Hello!

I have a used 92 Passat CL with no drivers manual anymore. The fuel
opening area say 87 Octane is fine, but premium fuel is preferred.

I assume there must be some knock sensor to detect grades of fuel?

Is there any benefit to using premiun (91,92,93 or 94) octane vs. 87
in this model of car?

Better MPG? The difference in cost between a litre of 87 and 91-94 is
$0.20/ litre - so it is significant (1 US Gallon ~= 3.8 litres). I am
trying to see if is worthwhile

Right now in the city, a 70 litre tank of 87 gas gets me around 600km
in the city, around 800+ km on the hightway. (1 mile ~= 1.6 km)

Thanks,

Tmuld.