As is very common, we don’t know the total number of miles that the car was driven between oil changes, whether the oil level was ever checked during that period of time, and what type of driving conditions the car is normally subjected to.
If the car was driven in mostly highway situations, the damage may be minimal.
On the other hand, if there was a lot of short-trip local driving for thousands of miles, then there could be a significant accumulation of sludge in the engine.
In the absence of that information, my advice would be to follow edb1961’s advice, and to do a few 3k interval oil changes in order to–hopefully–clean out some of the sludge that may be sitting in the oil passages. Then, make sure that you fashion some kind of reminder sticker that you can paste on the upper left corner of the windshield in order to remind you of the mileage and date for the next oil change.
And, of course, make sure that you monitor the oil level at least every couple of weeks.