I wouldn't change your oil after using it that long...just me!
Haha. I read this thread a second time. And noticed that you suggest to NOT change the oil. I know what you meant. I just found it funny "verbatim".
But yes. I agree with B McC. If the oil brand you have been using all this time works, don't change brands. Also, as odd as it sounds, some engines, after years of using one brand/weight of oil, seem to react badly with a change. As in the rings and bearings tend to wear rapidly after an abrupt change. I personally don't know this as fact, but it is what I have heard from companies such as Mobil. They could have an agenda. I don't know. But, again, don't fix it if it ain't broken.