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Wow, that's got to be one of the worst plugs I've ever seen. How does it even spark? I'm fairly sure that's caked oil, not unburned fuel. Uniform 150 psi compression is great. Other plugs all normal says it's something specific to that cylinder.
So where's the oil coming from?? Not the rings. 10W30 or even cheap brand oil hasn't got anything to do with this. Volvo specs middle weight 10W30 for normal operation anyway, but there are good reasons to go with 10W40, such as higher mileage or severe operation.
Any chance that plug recess is collecting oil from a leaking valve cover or spill? If the plug had loosened (which I've had happen when torqued to minimum spec in my 740 red block 16V head, dealer service knew of this tendency) then it could be inhaling oil during running, especially when cold before heat expansion might seal a plug leak. Unlikely, but in theory possible.
No matter what, you need a known good plug in that cyl before going further. As a temporary measure for testing, you could try cleaning it up or using an old plug, just be sure to use one of the other good plugs in that cyl for testing, not the old one. Check the ignition side. Is the plug wire seated firmly in the dist and also gripping the plug stem same as the others? Test spark against the block under dark conditions. Nice bright spark, same as all the others? If all that checks out then I'd maybe start up and if all is running normally then try using it for a few days if you can, then read the plugs again just in case the plug was the issue. Meanwhile you can think of other options like a leaking injector or some kind of valve or worn cam issue.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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