with 156k on it I don't see a problem due to the fact that all your bearing clearances are larger now due to 156k miles of wear.In fact, a lot of oils out there targeting high mileage vehicles are most likely thicker oils to do just that. Do you know what your oil pressure is? If it is lower than most engines you had it won't hurt. The biggest thing with the thicker oil was when the engine first starts cold and it uses the high idle immediately the thicker oil did not make it to the cam lobes fast enough and that would wear the cam lobes round as evident with GM's cam lobe issues in the early 1980's. Their fix was thinner oil, 10w30. If your engine was as tight as new, just use oem spec. due to 156k, and your average temp. outside, if it is hot mostly use a can of stp or just use 10w30 or even 10w40 in it. I do on my high mileage gm.
mike
