When I got my ‘07 335 in 12/06, I asked the service manager if I should change the oil at 1,200? The M cars get it. He said if I owned your car, I would. If I leased your car, I wouldn’t. So I got it changed and paid $68. I didn’t do the rear diff since a coworker
told me I have 1WD.
You are correct on diminishing returns. With my Toyota product there is a published chart and it’s recommended that 3 drain/fills are done. First leaves 55% old, second 31% old, third 19% old. Fourth was like 11% but unnecessary.
I hope all can see the pattern. It can never reach 0%, it can only approach.
They say stop after 3. Assuming it’s not spent when this started, Toyota Motor says to leave 19% old fluid.
Can you imagine doing 3 drain and fills every 15k? Even Ohtani isn’t gonna waste money like that.
Honda had an automatic back in the 90s to early 00s that needed more frequent changes to help prevent failure. Eventually they lost a class action lawsuit on the failure rates and had to extend warranty to 100k. Just an anecdote, but I don't think we can just assume the correct intervals.