Gandolf - first - Welcome to BITOG!
Next, I'm moving this thread to passenger car motor oil, since it's really about oil, and not a site sponsor question.
Finally, I own three of these. Including a 2001.
The P2 chassis Volvo is a great car with a couple of weak spots: PCV system, control arm bushings, and, in 2001, transmission valve body.
Kira's question about the stop-neutral is a good one. Dumb idea, increases wear, should have been removed during a dealer software update in the last couple decades...
For oil, use a good ACEA A3/B4. I've had excellent results with Mobil 1 0W40, Pennzoil Euro 5W40, and Castrol 0W40. All good choices. UOAs posted on BITOG over the years. The Euro spec is more important than the viscosity. You're looking for a long drain interval oil.
Flame Trap: this is the PCV system. At your car's age, with unknown mileage and service history, it's a good bet that system needs complete replacement. Neglecting to do so, or trying to cheap out by only replacing some parts, will result in a camshaft seal failure from increased crankcase pressure, and that requires a complete timing belt job along with the PCV system.
Timing belt: do this on age alone. Buy the complete set, including tensioner and idler pulley.
IF it shifts OK, then consider yourself lucky. It may have had the valve body (control unit) replaced a while back. Volvo did a recall on this years ago. If not, then a control unit (Volvo speak for a valve body) early will prevent the damage to internal parts. I retrofitted a new valve body to the 2001 - it was a GM part for a Saturn, but new production. Required a solenoid and some wiring work. On the other two, I had the existing valve body rebuilt. My wife's XC has the highest mileage, about 260,000, on the original transmission, with the rebuilt body. Good transmission if you get the valve body sorted out.
Changing the transmission fluid (all of it, using a cooler line flush) is a good idea. Use a Toyota T-IV (JWS-3309) fluid. Your choice, but the Toyota dealer gives me a break on a case price and you really don't need miracle fluid, just new fluid.
I've retrofitted an oil cooler with thermostat and external filter on my cars. Note that the factory filter on the AW-55 is internal, not accessible, and can only be replaced by removing the transmission and splitting the case open.
Details on all the above if you're interested.
In the meantime, here are some resources:
Transmission flush discussion and some pictures of the cooler, thermostat and filter installation:
https://www.volvoxc.com/forums/showthread.php?27334-02-XC70-Transmission-Flush-From-Bottom
Valve body "how to":
https://www.volvoxc.com/0/resources/how-to/pdf/2002-V70-XC-Valve-Body-Replacement-Notes.pdf
IPD kit for transmission flush:
https://www.ipdusa.com/products/4808/107945-volvo-automatic-transmission-flush-hose-kit-ipd-107945
IPD kit for PCV System replacement:
https://www.ipdusa.com/products/498...0-s60-s70-v70-xc70-turbo-genuine-volvo-114177
IPD kit for Timing belt:
https://www.ipdusa.com/products/516...nsioner-and-idler-bearing-oes-112855-30758261
Cheers,
Astro