1) As noted the flame trap used by Volvo in place of the ubiquitous PCV one way "valve" which has been used on numerous cars since the caveman 60's. It simply allows the crankcase vapors to be consumed in the combustion process, but not allow any mistaken backfire "flames" to explode the crankcase.

2) If your 850 is a non-turbo access is very easy, it's covered quite well in the cheesy Haynes manual as I recall, well at least there should be pictures, etc. Also check bay13 for instructions. Combine this with these posts while looking down at your throttle body area and you'll go: "Bingo - easy job!!!"

3) I looked at mine for the first time at 50,000 miles. Spotless, with a thin film of oil. You already got a warning on this, but this is a huge benefit of synthetic oil. Synthetic oil just won't crud up and plug the flame trap...because most of the crud is hardened oil additives such as viscosity enhancers.

4) So why all this about something so simple? Well if the sucker clogs up, at best you'll have oil leaks...at worst you can actually blow oil seals.