As stated the Mobil 15w50 has been superseded by their new 10w60 high mileage oil. 10w60 is a bit thick for winter use in my view and if your not tracking or driving the car hard the thicker oil is a bit unnecessary.
There seems to be a lot of misinformation around oil and its suitability. To say an older engine can not benefit from modern oil is not true, whilst you may have to be a little more careful in what you put in, a good quality synthetic oil has many benefits beyond longer service intervals. In a car like yours, not used on track or driven hard 90% of the wear of your engine is going to take place in winter and cold starts. Therefore theoretically you need the thinnest oil you can get away with to circulate the engine quickly in low temps, good quality thin oils like a full synthetic will perform better than thicker cheap ones and will stay in grade long after the cheap oil has broken down. The issue you may face however is that oil consumption may be higher with a thin oil, 0w40 is a bit too thin imo.
Other benefits of true synthetic oil is that it is highly polar, meaning it clings to the metal surfaces and protects better from cold starts, also good quality oils will have good additive packages, these help keep your engine clean, deal with fuel dilution, stop the build up of dangerous acids, prevent sludge, and should contain decent amounts of ZDDP (required for some older engines but removed from some newer oils).
Compared to other running costs oil is cheap but has to stay in your engine for 12 months or so dealing with all that you will throw at it, my thinking is invest a little more in oil and it will be able to handle the next 6000 miles with ease before your next service, put in a cheap 10w40 and it may have become a 0w30 even by 3000 miles, at that grade it wont have the capacity to protect your engine properly.
At sub 100,000 miles you car is relatively low mileage. If you are using the car in winter I would consider a good quality synthetic 5w40 with Acea A3/B4 rating (Millers or Fuchs are very good) and see how your oil consumption goes, if its higher than youd like then a 10w40 may be better.
Edd
