I will get flamed for this but I have to give my opinion. An 18 year old car with 78K miles on it averages 4104 miles per year, 342 miles per month. At 5000K oil changes that comes out to 14.6 months per change. That also comes out to one tank of fuel per month. One year on dino oil was way to long. It should be changed at least 2 times per year.
If I had just bought that car, I would use Mobil 1 oil syn and change the filter, AND the oil at 1000 miles for the first 2 or 3 changes. That should get it cleaned out and ready for a long life. I have a 91 D with K on it and it just got back from a 1000 mile trip and didn't use any oil and got 34 mpg average. I guess I go overboard when it comes to maintenance but I wouldn't be afraid to get in either of my cars and start out across country. Combined they have over a half million miles racked up.
Paul

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84 500 SEL (307,xxx miles)