I could fill this entire website with opinions, science and anecdotal evidence about what grade, brand and type of oil to use in your XC90. But I won’t. Instead, I’ll tell you what I use:
Mobil 1 synthetic 5W-30.
That’s it. Volvo suggests that grade, so I use it. I choose synthetic because I believe it’s superior to conventional oil. I don’t use additives, cleaners or anything like that. I have used Seafoam in a previous Volvo I owned and maybe it was a coincidence, but the cam seal leaked shortly afterward. I don’t know if the Seafoam dried them out, shrunk them, or if it was just random, but I’ve not used Seafoam in the XC90.
I change my oil about every 4,000 – 5,000 miles right now because I find it gets dirty very fast thanks to what I assume is buildup from previous conventional oil changes at dealer-recommended intervals. I’m hoping this doesn’t last too long, but I’m willing to put in the extra time and money to change the oil more often as it gets dirty.
If it continues to get dirty as fast as it has been, I may consider adding Seafoam just to speed up the cleaning process, but I’m not at that point yet.
Others will tell you to use high mileage oils, others will tell you to use different grades because of the turbo and small passages, etc. I feel safe using my choice.
As for filters, buy a decent filter. FCP Euro has decent filters. I find it hard to find filters in-store locally (in Canada) and if I do find them, they’re often expensive. It’s like some sort of import tax or something. So I usually buy a handful and pick them up every time I go across the border.