Please refer me to this post. (link?) I did read the post about someone that switched to Mobil 1 and the valves were noisier.

Why, oh, why were you using Castrol dino 20W-50? 150K is real nothing major for a NA Volvo. I have 230K+ miles on the 85 245Ti. But if you want your car to really last you should at least use the viscosity as recommended by Volvo....I mean a synthetic 20W-50 typically has a low enough viscosity to pump fast and early...but a dino oil 20W-50 even in mild climates, just takes TOO long to pump and lubricate. Also the 20W-50 clings to parts longer is pretty much a myth, too.

If I were you and wanted to switch, indeed I would worry about the leaks starting -I can almost guaranty that even a 3000 mile change intervals there is crud in your engine and crud wearing at your seals and to some extent slowing/stoping leaks. I would do a good flush. And the first batch of Synthetic oil should only go 3-4000 miles.

As to what weight: 10W-30.
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