I've never heard of an "oil trap". There is a flame trap. See
this site for general maintenance inclduing the flame trap:

http://volvospeed.com/maintence.htm

Note that some dealers/techs remove the flame trap so it may
not be there once you get it all apart. By the way, turbo
models do not have a flame trap.

The manual calls for 10/30 oil. I use 10/30 Mobil 1 in my 850 and
98 S70. Use only Mann or OEM filters. www.volvoparts.com sells
discount parts including the filters. I change oil about every
5K miles but you can go longer or shorter depending on how
the car is driven.

You will need to replace the aluminum crush washer under the oil
drain bolt. Some dealers will sell you a copper one. You want
the aluminum one. (Most dealers probably remind you of this when
you purchase a filter from them.)