Check engine light - Car running too lean
 

2016 S60 Inscription with 75,000 miles. Check engine light came on. Autozone and O'reilly diagnostic device indicates something with the air / fuel -- running too lean. They had different readouts on the part probably because they don't know Volvos well and don't sell the part. My experience while driving is the idle does fluctuate / engine rev while stopped at intersection in drive gear. I called dealer for a part and they were not helpful. Anyone have any experience with this?

Check engine
Idle Air Control Valve
PCV Valve

Does this have the 3.0L I6 engine?
If so, likely your PCV is pooped.
This guy has a good video on how to change it.
I have done it myself and it's not hard. I have some absurd problems going on that may or may not be related to this but it is a good first step for you. Just the cover+valve is not expensive.
Could also mbe a MAF sensor, do you have a scan tool with which you can view live data?

usually lean burn fault codes are caused by an intake air leak (even though some CEL codes on older models suggest it may be an O2 sensor). Several things should be checked out before dropping in parts - inspect vacuum lines, check intake air tubing (ie spray carb cleaner), PCV oil separator and exhaust/manifold leaks are also on the suspect list. Never done a oil separator on a Volvo but on a VW there's a check valve that can get sticky and can be cleaned (requires removing the oil separator). You can try cleaning the part and inspect the gasket if you have the time/skills/tools - there should be vids on Youtube on how to. Note that looking for an intake air leak may require a smoke test - not all tubing is directly visible.

Curious what part did you call the dealer for and how did you determine which part needed replacing? Note, once you get to the point of ordering parts, you can always use a dealer web store to find the OEM number and genuine Volvo parts. I am in the northeast, so I like to use Tasca Parts but there are plenty of others - and the online prices are better than walking into your local dealer.