Learn to sharpen your chain, and buy multiple loops and sharpen at home if touching up frequently in the field doesn't appeal.
Some of the stuff we cut here needs a touch up every few cuts when crosscutting (bucking) reasonable sized logs.
It depends on the silica content of the wood, how dirty the bark is (whether a skidder or dozer has dragged it) the type of timber, whether termites have gone up the guts and made a mud chimney, etc.
Hit dirt and you cutters need a touch up with a file, hit a rock and it's a visit to a grinder.
As soon as you start to feel the cut speed slow down it's time for a touch up, don't keep pushing through, all you'll do is burn up the chain and bar, or throw another chain on and sharpen them all at home.
As for chain, Stihl, Oregon, Carlton, Windsor are all good.
I keep reading on all the US boards that Stihl is 'best', yet most Aussies I know prefer Carlton or Windsor in our tough as nails timber.
A mate on another board did some hardness testing of the drive links and cutters of Stihl, Oregon and Carlton semi-chisel 3/8 chain and the Oregon and Carlton were line ball within the test parameters (within 2% of each other, Oregon slightly out pointing Stihl), with the Carlton sample about 3% harder.
Being a scientist he also timed the cuts when slabbing the same log with his Carlton and Stihl chains. Initially the cut speeds are the same, yet when getting towards the end of the cuts, the Carlton finished first.
He repeated this a number of times.
I can dig the links up to the other forum if anyone wants to read it all.
If you are fairly experienced at cutting, stay away from most all 'safety' or 'reduced kick back' chain as it is generally a fair bit slower than the 'pro' spec chain, although I'll concede there's a reason and a place for it's use.