Those Briggs and Stratton engines these days are pretty rubbish, so plan a decent service every year (and be dismayed at the plastic crap carb systems!) 
I filled it with half a litre of shell 5/30 fully synth which the book says should be changed after just 5 hours of mowing......why?
Hard to work out petrol use, but probably about 5 MPG. 
Lol, 5 litres of fuel tends to last me a season on my Atco, it runs on GM fully Synth oil. The short interval first change will be due to the running in and getting the 'bits' out before they do further harm
At the end of the season ensure you empty the tank and run it until it stops so the fuel bowl is dry
I hear conflicting points of view on this.....
My Dad used to have a petrol rotavator and he did this every year, after a couple of years it wouldn't start....when he got it repaired (new carb required) he was told never to drain the carb and leave it with fuel in over the winter.
My 10 year old plus Stratton Briggs mower, I have done the same....left it with fuel over the winter.....never a problem...except last year, when I replaced the plug/air filter and carb gasket to fix a miss fire....first bit of maintenance it had in its life....still with original oil too
