The thick BMW 5w30 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, synthetic oil is better if that means over 5000-6000 miles. If you like changing it ever 3 or 4 months, maybe a plain conventional 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 conventional 30 weight might work in her case, if it's a good blended Semi Synthetic with the Euro specs of A1...Castrol Syntec Blend 10w30, 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 5w30 which flows like a thin oil but protects like the thick 5w-40s.
Many people here mention Auto-Rx, it works great with the conventional 15w40 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 10w30 in winter is an IDEAL combo and conventional oil will serve you well (Motorcraft, Pennzoil, Havoline). Add some Auto-Rx and you are SET! Even if you decide on synthetic oil, it's best first to get the engine clean a bit for the synthetic oil to work right.
http://auto-rx.com
