But people that run it down to almost empty is more likely to run it dry if they can't get to a gas station in time, whereas people that fill up when it is down to 1/4 tank is less likely to run it dry.

I don't see how filling at 1/4 of a tank can creak a leak at the siphon jet pump. It would be better to blame the engineers at BMW for designing a siphon jet to transfer gas from one side of the tank to the other side of the tank instead of just designing a tank that isn't split into two tanks.

Or blame the BMW workers at the factory for a sloppy job of fitting the 0-ring.

But it is easier to blame a wife because she is in the habit of filling the BMW at 1/4 tank instead of running it down to almost empty instead of blaming yourself for not putting in gas when the gauge is down to 1/4 tank.

You missed the point entirely. If you always fill at 1/4 tank you will mask the fact that the siphon jet has failed. You won't realize that you CAN'T use the last 1/4 tank until it's too late. The false sense of security doesn't help you much.

Laws of physics dictate what I am saying.

On the X5 there is simply no difference between 1.4L of gas on the gauge and 30L of gas unless something is broken.

Feelings about the topic won't change the facts. There is not one cc of additional fuel either around or going through the fuel pump to cool it once the tank gets below 1/3 of a tank. The pump sits in a well that holds 5L of gas only. Any extra spills over to the left side.

The only way to know if the MFG defect of the badly designed siphon jet o-ring has failed is to drive the car down to near empty and monitor the liters of gas in the tank.

When the system is working, you can safely drive down to about 7 miles distance to empty with no possible additional wear on your fuel pump and If it's not working it's better to find and fix the problem rather than find out the one time you drive lower than 1/4 tank because you are on a road trip and want to go more than 300 miles without stopping.

Although the principle of keeping enough gas to keep the pump cool still applies, the amount to do that is NOT 1/4 tank on the X5 it is 1.4L on the gauge. You definitely want to keep at least 1.4L of gas in your tank to make sure the fuel pump is properly covered and cooled. Any more than that simply spills out of the pump well and makes no difference to the system whatsoever.

Now technically, since the unused gas does go around in circles from pump to FPR to the left tank back to the right tank, there is a tiny advantage of having more liters of fuel to warm up but it will only amount to a few degrees in temp because by the time it would cause a problem, you would have to stop for gas anyhow and have a fresh tank of 55° fuel to keep things cool.

Fact: the o-ring design of the siphon jet is piss poor. It makes no sense to have something made of plastic that holds 18 psi and only held on one side it's a doomed design, a fact proven by the countless examples of identical failure you can find online.

Fact: once the siphon jet o-ring fails you will not know it fails until you try to drive to a low DTE.

Fact: the right side of X5 tank only holds 5L of which only 1.4L shows on the gauge. At 0.0L on the gauge the pump will still be completely submerged

All rational for keeping more than 1/4 of a tank is psychological. I'm not saying there aren't very good reasons like the guy above who can use the gas for emergency power generator I'm just saying the the 1950s rationale that is presented by consumer reports may have applied and may still apply to other cars but doesn't apply to the X5 because of the way the tank is designed the "1/4 tank" literally translates to the "1.4L rule".

The X5 fuel can't tell the difference between 1.4L and 28L as any more than 1.4 on the gauge spills out of the right side and won't add 1 mm of additional fuel above the pump.

If I'm missing some scientific fact I would love to know if I'm wrong but that's not the way to bet, especially when using ethanol polluted gas which removes any water and extends the fuel pump lifespan.

So; for those not confident their siphon jet is working, put a portable tank of gas in your trunk, set up test six and drive down to single digits to confirm your fuel pumps are working and grow some confidence or discover your siphon jet is leaking and fix it for a couple bucks. Once you know your pump is working you can quite safely run down to the low fuel light every tank (8L/10L on the gauge, 11.6/13.6L actual).

It doesn't help you in the long run to hide when your siphon jet fails, as it will slowly strand you with a higher and higher distance to empty. It makes no sense to keep extra unused gas in the tank rather than fix the prob and be able to use your full tank.

Once you know your siphon jet is working than by all means for any of a bunch of personal reasons keep your tank with as much gas as makes you comfortable. filling at 1/4 or 1/2 a tank will be absolutely more inconvenient in the long run but if the trade off and the false sense of security helps you sleep better more power to you.

Fact: when they do long term tests on fuel pumps the fuel is pumped in circles out and into a 5gal bucket for months, no cooling needed other than it's in a lab perhaps 75-80F. Even when down to say 10-15L of gas circling around for the hour or so before you need to get fuel, there is virtually no chance that little bit of extra heat will cause any harm.

I'm not exactly sure what the argument is: it's absolutely fact the pump on the X5 is completely submerged when there is 0.0L or more on the gauge, so to the CAR there is no advantage to have more than that for the fuel pump.

That all said for DIESEL that's a different story as water sinks and if there is more diesel swirling around from right to left then hopefully it gets diluted more when more fuel to get sucked out and consumed. If I had a diesel I would probably regularly refill at 50-60 miles and only run down to single digits once a year to make sure siphon jet was working. The less fuel the bigger odds the water % gets higher.