Hello Peter, I haven't heard from you in a while,

Full Synthetic is the way to go, it's made from natural gas instead of crude oil and doesn't have the impurities common to dyno oil that clog up engines.
Mobil-1 tech says use 5W-40 Euro blend in my B230FT.
Castrol and Penzoil are running summer specials, Mobil-1 $20 off ended Aug 31st.
see here for the Castrol promo:

https://www.brickboard.com/RWD/volvo/1689642/940/960/980/V90/S90/castrol_promo.html

Summer is the time to stock up on Synthetic oil.

I got my 945T two years ago and it was always run with dyno oil.
I switched to Mobil-1. In short order the oil turned a very evil black
as the synthetic was cleaning out the mess from dyno oil.

I don't like the idea of a harsh engine flush and the chance to dislodge some evil chunks of gunk in the engine to maybe have it get stuck in some oil passages. A few changes of Synthetic will clean that mess out safely.
I changed out the mobil-1 and Mann filter twice in 3K miles/2 years and on the third shot of 5-40 this latest Castrol Edge API SN/CF is staying clean now

If you have a slow weeping leak you can have luck with Rear Main Seal (RMS) treatments. One local pro mechanic said that a capful (1 tsp) Maximum of brake fluid in the oil will work to swell leaking oil seals.
That's about right, go to the local auto parts and buy RMS treatment: a quart of oil with 1 tsp. of brake fluid for $12.00 + Tax

That reminds me of getting this chemical at the tropical fish store to reduce
acidity. They never told you it was just soda bicarb and sold you an ounce
marked up 2000 %. Business is business eh?

Yes a weeping oil seal weeps a bit more when you switch over to full synthetic,
but it stopped after about 1500 miles with the RMS stuff and full synthetic.
(This was in a 5 cyl 850 that needed major TLC to repair the damage of dyno oil)

Cheers, Bill