First, you've got to slow down.

You've got too many balls in the air and you're never going to find an answer to this problem until you put some of those balls down.

And you need to establish a baseline.

First of all, I don't have an 850 with an oil level light, but in reading past posts about this feature, it sounds very unreliable. I guess the fact that Volvo discontinued the feature should give you the hint that it is not something that should be relied-upon.

So, forget about the oil level light. Make believe it doesn't exist. This shouldn't be a problem 'cause it sound like you know how to check your oil and it sounds like you currently do it frequently-enough.

Next, check your oil when the engine is cold (every morning). Record the reading. Over the course of a month, you will have an accurate record of your car's oil consumption. You can top-it-up each time, if you'd like. Regardless, you will have a consistent record of your 850's oil consumption. DO NOT do it, one day, first thing in the morning, and the next day, at the gas station after filling up. Do it while the engine is cold, first thing, every morning.

Once you do this, you can come back here and you will have facts that we can all use on which to base an accurate evaluation.

The shops that said give it a tuneup, loose them.

As to 20-50, is that what is recommended by Volvo in your owner's manual? I don't think so. Was it recommended by the guy you bought the car from--possibly to hide an oil consumption problem? Change the oil to a recommended weight and then conduct the test above.

By the way, shell bearings suffer their greatest wear damage on first starting up. Heavier weight oils aggrevate wear on startup. So heavier weight oils may reduce oil consumption, but they do long-term damage to your engine.

As to flushing and all that other stuff, first, let's see if there is a problem.