I wouldn’t make any assumptions on the thermostat being the cause of bad mpg until you know for sure your turbo vacuum system is 100%. Bad glow plugs are much easier to diagnose without a dpf and scr filter
. Bad thermostat is very common though. If you have a scan tool it would be good to check to make sure your coolant temp gets up to atleast 180F. I think there’s also a way to get coolant temp on the gauge cluster when you first start the x5. I have a banks gauge in my x5