The thick BMW 5w-30 Castrol is a superior product. People have it on EBay cheap. It's good for extended oil changes, if you desire that. I tell people to first decide how often you WANT to change oil and go from there. Notice I said how "often" not how many miles. That's the second question. So, if you only want to change every 6 months or a year, synth oil is better if that means over 5000-6000 miles. If you like changing it ever 3 or 4 months, maybe a plain dino oil (10w-40) is fine IF NOT BETTER.
I would also say to experiment with different oils, but since it the wife's car, maybe it's not worth worrying about. I put good oil in my wife's car, but it's simply the stuff I already have and don't want for my car. I suppose she does a lot of lightly driven short trips, and the occasional hwy blast to the Mall? If so, thinner oil will serve you better. Even a thin dino 30 weight might work in her case, if it's a good blended semi synth with the Euro specs of A1...Castrol Syntec Blend 10w-30, for instance.
I also say to observe the oil temps, if she rarly cracks +100c (+200f) a thicker oil is not needed at all. If so, the Shell Rotella Synthetic 5w-40 is among the best 5w-40 oils. If you feel adventureous, grab the German Syntec 0w-30, a thick 30 like the BMW 5w-30 which flows like a thin oil but protects like the thick 5w-40s.
Many people here mention Auto-Rx, it works great with the dino 15w-40 that is good for summer. It should do wonders for the engine. Take her car out yourself and run it hard to heat it up with the Auto-Rx in there nd it'll melt away any deposits. After that, it's actually hard to go wrong if you match the oil temps to the visc (hotter=thicker oil) and the "w" ratings to the seasons. Anything over 10w is not good in Northeast winter.
10w-40 summer and 10w-30 in winter is an IDEAL combo and dino oil will serve you well (Motorcraft, Pennzoil, Havoline). Add some Auto-Rx and you are SET! Even if you decide on synth oil, it's best first to get the engine clean a bit for the synth oil to work right.
http://auto-rx.com