First of all, the 960 wheels have a different offset which will not work well with the FWD 850, sorry.

As to a bad headgasket, your mechanic was almost correct. You don't need a blend of oils, just basic motor oil. Pull the bad cylinder spark plugs and using a turkey baster, pour a table spoon of oil in. That will help coat the rings and should give you enough compression to "start" the engine. You only need to wait 5 or 10 minutes before restarting. Remember, the compression readings on a cold engine will be very low because of the lack of oil on the rings. You should have close to 180psi each for a new engine.

Don't worry too much about gasoline in the oil just yet, it will thin out the oil and lower the oil pressure.

Headgasket failures between 2 & 3 are common and usually due to overheating or over boosting in your case. I would opt for a rebuilt head rather than an entire engine because it is an R. About the same price if the valves are inspected and new seals and guides are installed.

Klaus
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Common sense trumps a PhD