E39 not like e53. Completly different system.
There is absolutely no difference in how deep the fuel pump is submerged once the tank has > 5L. The design is completely different from road cars. It's designed to keep the pump submerged when going up and down hills and keep the pump submerged when low on fuel.
"Below 1/8 risky" is absolutely incorrect when referring to the E53. In road cars that may still apply. Last time I checked this is the E53 forum where that rule does not apply even a little.
In addition, should you follow that rule (1/4 or 1/8 rule) you INCREASE your chances of being stranded. I know this from first hand experience!
My wife used to use the 1/4 tank rule. It masked the fact her fuel pump was worn out and left me stranded. The ONLY ways to test the function of the siphon jet is to either drive down to single digits DTE occasionally or add a fuel pressure gauge to the return line from the fuel filter and confirm you have enough pressure.
A siphon jet not working will cause a fuel starvation problem with fuel in the tank but it will not damage the pump UNLESS you ignore the problem and try to nurse the situation by keeping the tank full enough (again a case where keep 1/4 tank will cause a problem)
I've seen hundreds of posts about siphon jet and fuel pump failure. I've spent about 20 hours researching how the E53 fuel system works.
I found not one single page that describes the fuel system accurately for the E53 so I literally "wrote the book" with the singularly unique but accurate depiction of how rhe fuel system works on the E53. Literally the only accurate description online that I've seen.
The sedan fuel tanks are different: the lobes are similar left and right. There's a baffle to keep fuel near the pump on hard g-forces but completely different from the x5 which has an actual sub-tank that holds about 5L of gas and will hold exactly that from 30L of gas until 5L unless the main pump or siphon pump have failed. What that literally means is that the x5 does not know the difference between 5 and 30L of fuel.
There are no moving parts in the siphon pump, the only failure points are either a seal or the over pressure relief valve.
After reading hundreds of posts about such failures I determined that the electric pump failing is nearly universally the cause.
90% electric
5% siphon
5% something else (Filter/FPR)
Even when there is a problem with the siphon (leaky seal) once the weak electric pump is replaced, the siphon will work again.
That said, when I ever replace an electric pump I will check for leaks on the siphon side because it's a crappy design with one joint having no actual o-ring seal.
To repeat: zero advantage of keeping more than 1/8 tank of fuel in the E53 only adds inconvenience and more likely to mask a failing fuel pump and increase the chance of being stranded.
If you "always" fill at 1/4 (1/8) tank you will absolutely get stranded because of the practice eventually because you will mask the failing pump and the DTE at engine stall will slowly get higher until it rises above the percentage you arbitrarily decide to fill.
If you on the other hand as I do, drive to the low fuel light you test the siphon jet every tank and will know right away when your pump dies.
My wife's pump was broken for about 9 months before I found out at 6°F on the freeway because she used the 1/4 tank rule. Had she not followed that bad advice I would have known in April not December and could have changed the pump when it was 60°F vs 0°F
e53 Fuel Supply Detailed (fuel pump / siphon pump diagnostic help)
https://r.tapatalk.com/shareLink?sha...6&share_type=t