Do an Oil change every (insert engine service hours) or 12 months, what ever turns over first. also for the sake of a few extra quid change the filter every service.
if its a little over then dont panic but dont leave it years, oil degrades and loses it's full protective factors when exposed to air and engine impurities.
my lister TS2 had not had a proper service for some time when I purchased 'Tipsy Vixen' It didn't even have an oil filter, she run but rough as a badgers arse!
She had a flush oil run (clean cheap oil or the right grade) a cheap filter at the cost of £3, blank plate was removed and filter fitted, run for some hours then drained of oil, cheap filter removed, clean "fairly" good quality oil refilled and normal quality oil filter refitted. she runs beautifully now with hardly any smoke, quieter too.
on chopping up the cheap catch filter, it was full of oil sludge black carbon bits etc etc but thankfully no metal fragments!
you shouldn't need to double up on the filter with yours but if the oil is really bad consider a clean flush and run first, then after that just log your services in a little service book.