Last year I went 53,000 miles before changing the oil. The vehicle is a 1999 Dodge Grand Caravan 3.8 litre motor. It had 350K on it at the time I started this super long oil change. The engine also has a Motor Guard bypass on it.
At the start of this oil change, back on Feb 15 2008, I used Amsoil 10-30. As the miles went on, I heard about Pennzoil Platinum so about every 5-7K I would change out the TP in the bypass and top it off with the PP. I would burn about one quart every 5K so each time I changed the TP I would end up adding 2 quarts. The non bypass filter was the Amsoil EAO which stayed on for the whole 53K.
When I checked the oil when the engine was hot, it looked almost clear but when it was cold it looked dark. I did an oil analysis at 53K and the oil was still good. I have that around here somewhere but I have no idea how to post it here. The TBN was still good and I probably could have gone on like this for as long as I wanted but for some reason (not sure why) I decided to change it.
I'm currently doing a slow flush with MMO and I have to say I'm surprised that this seems to have breathed a little life into the engine.
Today I will do another change and fill it with PU. How long this will stay in, I don't know, but with a bypass, I'm thinking that oil can last almost indefinitely provided the TP is changed out every 5K and new oil added.
The vehicle now has 416,000 miles on it and it's still running just fine. Original tranny too! Yup, 416K! Probably a world record for a Chrysler tranny.
Oh, I have to thank everyone on this forum for all of the great info. I'm sure it has helped me to make my beloved Caravan run better, last longer and it's been fun on top of all of that.