Without knowing what the consumption is, and where you live (i.e. the
ambient temperature), it will be difficult for someone here to give you
advice.

Our 850 consumes about 0.5 quarts of oil with 5W30 (during the winter),
and almost nothing with 10W30 in the summer.  Very similar mileage.  So
a heavier oil may help you.  Offhand, my recollection is that Volvo doesn't
recommend anything heavier than 15-weight oil in this engine.

Beverly

I would look into the cause of the consumption, at that mileage it shouldn't
really be using any significant amount. Is it leaking? If not it could be as
simple as valve guides or a clogged crankcase vent.

My '95 850 with 125k miles uses less than .5 qt every 10,000 miles.  My
'93 850 used about .6 qt every 10,000 miles at 180,000 miles.  So oil
consumption is not normal.  I have had leaks a couple times, but the
dealer always finds them and fixes them.  I always service the flame
trap religiously.  I would not sacrifice the lubrication, but rather
find the problem.  Why sacrifice the engine to fix a leak?  Oil is much
cheaper than engines.

In other cars I have used 20W 50 with no trouble, unless it's under 15f or
so.  The heavier oil is a better idea in my opinion then additives.  Be
aware that the heavy oil will not protect as well for the first few seconds
a a 5 or 10w however.

Hi, thanks for this guidance. I don't know all that much about engines so I
am not sure why oil loss correction solutions depend on whether the problem
is through valve guides or past the rings? Do you know if there are other
items beside the flame trap, which I do not have, that can affect crank case
venting such that extra oil loss occurs? The EGR valve has been mentioned.
Are there other parts that need to be cleaned or changed?
    Thanks, Brad

try slick 50 it worked for me.