Originally Posted by JCL

The material that you take out of the pan, which most term 'sludge' is actually sitting on the dirty side of the filter, not hurting anything. If the filter is clogged, then that is a problem. However, the junk usually just sits in the bottom of the pan below the filter.

Parts don't break down with age. There are no higher pressures or stresses on the springs or valves. The fluid does deteriorate with age, as it oxidizes, but the question is whether that is enough to make a difference. The fluid doesn't do much lubricating in any case, it is just a 10w oil, so the most critical thing is that it flows properly. That is a non-demanding application for a fluid.

What happens is that deposits and varnish form in the transmission from age and heat. They are essentially baked in place. When you put in a new fluid, which has a high level of detergents by design, it cleans out all those passages. It then moves that crud, and can cause blockages in the valve body, solenoids, actuators, etc. It goes from where it wasn't hurting anything, to where it causes bad shifting by blocking flow. The new fluid is essentially a powerful cleaner.

BMW says not to change it because the balance of costs of changing it vs any benefit in increased transmission life, isn't positive according to their calculations. It doesn't matter who is paying those costs.

If you change it often, from new, then there is never much of a chance for deposits to build up. That makes it safer, overall. However, it doesn't address the question of whether the old fluid is hurting anything, it just means that changing it has less risk.

It is your decision. However, many here would not touch it at 150,000 miles and with no problems evident. Others would do it. There isn't a single right answer.