The performance on an 2011 opel insignia has gone from 5,5 ltr per 100km to 6.3 per 100 km ... recent service oil change filter change ... even blew out the particle filter... any solutions

No but check with the dealer or your service/owner manual for the manufacturer recommended gearbox oil change periods.
There is no filter on the manual gearbox .
Whatever the manufacturer recommended period is for a gearbox oil change, change the gearbox oil twice as frequently as recommended to maintain it in good condition. If, for example, a change every 50000 miles is stipulated then do it every 25000 or so.

This is regulated by the mileage of the car. When it gets time to change the oil. The message center will say "Change oil"

8 km per litre, or 12.5 litres/100km is certainly a bit heavy for that model, but not disastrously so, as in city driving it was rated at 10 L/100 km when new by Govt agencies where I am in Aust.

So the usual suspects in an older car are

- dirty air filter
- low tire pressures
- spark plugs worn or fouled
- ignition timing incorrect
- fuel pressure too high
- fuel injectors worn
- evap canister filter blocked
- emissions system faulty (EGR valve or Vacuum Switching Valve)

Probably a faulty injector

You would fill up the vehicle, then divide the mileage traveled by the amount of fuel it took to fill the tank again.
So assuming you filled the tank each time, you would divide 609km by 100.4 ltr which is .1648 or 16.49 L/100km
Thats about 40 mpg.

change the plugs and the air filter &try not to speed, that always consomes more fuel. if that dont help, change the plug wires

Published performance of the 2000 Dodge Neon is 27 MPG (Miles per US Gallon) for mixed City/Highway driving which translates to 8.7 Liters per 100Km.

Hope the information is useful.

From the owners manual 1998 model
manual transmission
city 12.8 litre/100km
extra urban 6.9 litre/100km
average 9.1 litre/100km
215 CO2 emmission (g/km)
auto box
city 13.5 litre/100km
extra urban 7.5 litre/100km
average 9.6 litre/100km
228 CO2 emission (g/km)

try changing the oil also it will cause the engine to burn more fuel with bad oil because it gets hot and evaporates it alot more

Not for the home mechanic, but you could try a fuel system service. Basically they add chemicals to your fuel tank via a machine, and these chemicals clean the fuel system... fuel injectors, lines, filter, basically everything... that seemed to help my fuel mileage and also throttle response.