Apparently you havent seen previous posts on subject and I think that whoever's looked at your car is not at all familiar to your Volvos engine. Why? There is NO valve cover for starters, it's an upper cam cover and it's not just easily removed to "take a peek" at the lifters in your engine.
If changing oil quieted the engine down at all (no matter what type oil is used) then I'd guess that your oil was either very dirty or gassed (too thin from gas dillution, running rich or being flooded).
The main reason why lifters make noise in "white engines" is due to low oil pressure. The main cause for low oil pressure on higher mileage or older white engines is because the oil pan (oil sump) o-rings have shrunk in time and thus you're sucking air.
If and when the lifter noise re-occurs, do NOT start with the valve train, start by having the oil pan removed and have the sump and two filter housing to pump o-rings replaced. Another area of possible pressure loss which I see with high mileage 850s is the oil pump o-ring, a large o-ring that goes around the oil pump and when it's getting bad you'll see oil leaking from the from crank seal area.
I've heard of several 850 owners who've paid good money to have cams and lifters replaced by independants only to still have the same "lifter" noise so don't be a victom of mis-diagnosis. It'll be big $$$ to replace valve train parts and a complete waste if the problem isn't solved. I have yet to ever replace cams and/or lifters in a "white engine" or ANY Volvo engine for that matter.