VW GACEL 88 Temperature problem

This weekend I change the thermostat, I want to put a colder one, or as they do in Venezuela, they do some gaps where you can (with a very fine drill, narrow as they want to say), so it circulates a little when the water when the thermostat is closed, what I want to know that it is wrong that the rubber above this superdura? The fan is aspiring well, I already have the system with refrigerant and those things clean, this temperature started the next day of the oil change, which I do not think is to blame, because I use the same oil and the same filter of other times.
Cuervomostro or Ponterablack01 Are you passion fruit? (by the fan)

To this point it is quital to the central part of the thermostat and it is already but remember that so the temperature in the cylinders will not be the same and when you have to do the engine you can get some oval cylinder, you will have the alfredo.

Try to disassemble the radiator and wash the honeycomb is often filled with dirt and does not allow enough air to cool the water