I would like to know how do you find this oil consumption after a top header gasket change?

I would also like to mention, that there isn't any leakage visible from the top or the bottom of the engine. I currently use synthetic blend 15W-50.

By the way what types of oil (viscosity and composition) do you use?

It's about a Sidekick 2door, 1.6, 8v engine, with 115.000 miles. I'm placed in an average clima, like 0 deg C winter and 40 deg C summer. (sorry, I cannot deal with that Fahrenheit thing)

Well, as for consumption I don't use any oil with 115,000 on my odometer.  As for viscosity, I use 5-20W (recommended) and live in a simular climate.  It doesn't quite get down to 0 Celsius here but I'm sure the 5-20W is okay for that too.  It usually doesn't change much until you get below freezing.  I think a 15W is too thick, that is my .02 cents.

What you are using is large Diesel Engine
oil, and it might be too thick for the scraper
ring to get off the cylinder wall completly, and
it ends up getting burned and used.

I would say 10-30 or 5-30 for the most part,
the temp rating in the manual calls for 5-30
thru about 40C  or about 100F

Synthetics seem to leak thru also, tho you
say you don't have a leakage problem. Try
a lighter oil, better in the cold anyway.

Thanks for your reply guys,

So today I changed the old oil with full synthetic Mobil1 5W-50. It already runs better I think, but the time will tell.