Been good. It's a BMW, what can I say. I knew this thread would bring you out. Lol.

Excited about this lower temp tstat. Will report after more research.

Why do you class 100C as the 'magic' number that everything 'bad' will be released and disappear from your engine oil?

Fluids dont need to boil to release vapour either (bear in mind engine oil will boil at around 300C). Also if the engine is running as it should and there's no issues with combustion then there should be very little with regards to fuel contamination anyway.

Moisture will be evaporated at much lower temperatures than 100C. Also it's more about the length of time the oil is at a higher temperature than the actual temperature itself.

For example - 30mins at 80C will mean alot more moisture will be burnt off than 100C for 1 min! I bet moisture will even start to get burnt off as low as 65C.

Ask yourself the question - why is the oil thermostat used an 80C one and not a 100C one for example - answer is because it doesn't need to be. If the oil is kept at at least around 80C operating temperature then everything that needs to happen will happen.

^this. I can almost guarantee that if you had 2 X5s that had done the same mileage under the same driving conditions yet the transmission fluid in one had been kept at least 20 degrees cooler than the other over say a 5 year period - the quality of the oil of the cooler running one would be higher.

I too also tend to change my engine oil every 6k - probably not required but I like to do it for peace of mind and knowing that the quality of the oil is as high as it can be - I like to drive my cars rather than just dawdling around in them too.

Gearbox oil I would quite happily do a flush every 2/3 years also especially if it means increasing the lifespan of the ridiculously expensive gearbox - after all - prevention is better than cure.

Spoke to a few people over the weekend that also told me that if you had the towing pack option fitted from factory when you bought the X5 then they fitted an external gearbox cooler as part of this pack which is interesting! This being the case BMW obviously didnt see any issue in running the X5 transmission fluid at a cooler temperature - I also doubt they altered their 'lifetime gearbox fill' policy either if you had one fitted.

I got mine from here...

Mocal Oilstat Oil Thermostat -8 AN male - 574-OT2F - Speedflow

Interesting bit of reading here - Everything about your ZF 6HP26 transmission issues | BMW Logic7 site

Granted it's for the 6HP26 gearbox but I can't imagine the 5HP24 characteristics would be much different if at all when it comes to heat/behaviour issues!

This immediately caught my eye.

Everyone also needs to keep in mind that in addition to oil/oil sump temperature being important for fuel and moisture cook off, remember the crankcase is also under a vacuum the majority of the time. Liquids boil/outgas at lower temperatures when under a vacuum or at less than atmospheric pressure.

So at the end of the day, the length or duration of the engine operation is more important than the absolute highest oil/oil sump temperature.

I will be doing this too. Never heard of it before this thread.

Think it will probably be the one out the 750iL/850ci

Part number - 11531704704

You'll just change the actual element so that you can retain the housing with the heating element in it.

Very tempted to do this myself to be honest.

As long as the element has the hole in the center for the heated post, it should work. Seems from the pictures I have seen the 750/850 stat may not be a heated stat?

One thing to keep in mind, with the 90C stat, you may trigger a P0128 code from time to time. I finally did, but my thermostat actually got soft and the operational temp had dropped about 8-10F from time to time due to the main spring getting weak. I just replaced the stat and will see what happens over the Winter as I have not run this stat through a full Winter season yet.

If someone can find a 95C stat, I think this wold be a big more ideal overall, but the 90C is fine until it goes soft.

ok so the part number 11537586885, it the same as your car. As a matter of fact, this thermostat is used across most V8 engines in BMW lineup.

The hamburg tstat looks like a direct swap to ours, even part number is same, other then the 90C at the end. 11537586885-90C

The 750 Tstat is also 90c, we are looking for a 95C one. The 750 tstat is also not heated. I don't think this will work, or worth while.

So far, the hamburg one is in the lead.

As you say....completely different type of gearbox coupled to a completely different type of engine which also should be worth noting runs substantially cooler. So really bears zero relevance to this post.

In fact the only similarity between the 2 gearboxes is that they are both automatic!

You'd struggle to find an original 5HP24 box in an X5 4.6is with 290,000 miles on it that's for sure!

Out of interest......what makes you think the 90deg one wouldnt be worthwhile (if it worked)?

Only downside I can see is that it might trigger a fault code from time to time.

Wonder if you could delete this from the map on the ECU so it either didnt check the temperature at all or perhaps if the temperature/conditions could be altered so it didn't register the fault in the first place!