The X5 is twenty years old this year. It feels at least that. BMW's first SUV pre-dates the Porsche Cayenne, and the original version - dubbed a "Sports Activity Vehicle" by its maker - was among the first of its kind to demonstrate that there was more to the modern 4x4 than the body-on-frame ignominy that preceded it. In 2000, to underline the point, the manufacturer went to the trouble of installing a Le Mans-winning V12 engine in one, and had Hans-Joachim Stuck pedal it around the Nurburgring in an unthinkably small time. Two years later, it fitted a 340hp 4.6-litre V8 to the E53, and sold the same point to eager punters everywhere.
By the time the E70 came round, the market was considered supple enough for the concept of an M-badged X5, and the result came with 555hp. Then, in 2013, with oil burner popularity booming, it happened upon the wheeze of the M50d, a model equipped with the 381hp tri-turbo version of the old 3.0-litre N57 unit. So addled were we by diesel fumes in those days, the idea of 546lb ft of torque at 2000rpm didn't seem all that unreasonable; nor the fact that, out of fourth gear, it made a 2.4-tonne SUV quicker from 30-70mph than an Audi R8 V10.
That was then though and this is very much now. In 2019, the prospect of a new M50d seems, even from exalted position PH occupies in the shrug-verse, a wee bit unnecessary. Porsche, after all, has abandoned the diesel segment entirely. And even if you're grimly unfazed by the buying public's waning interest in big oil burners, it's worth pointing out that BMW has not reproduced the same level of silliness here that Audi recently mustered in the SQ7, a model which could credit a brace of electric turbochargers (and a V8) for its knowingly barmy 664lb ft of torque from 1000rpm.
No, the latest 3.0-litre B57 motor, the M50d can lay claim to 'just' 561lb ft from 2,000rpm and 400hp at 4,400rpm. Its own party trick, of course, is the fourth turbocharger now plumbed in (two medium sized blowers having replaced the larger one that would previously come online once the lowdown job of two smaller ones was done). The result delivers a modest improvement in power over the previous generation, and ensures that it materialises at lower engine speeds. Technically worthy, yes - but actually worthy of a £71,475 starting price when a 313hp 40d or 340hp 40i can both be had for five-figures less and are no less able to break the 62mph tape in under 6 seconds.
Well, for what it's worth, the M50d lives up to its range-topping status well enough when you first slide in. Wherever it's not glinting or twinkling at you like a Swarovski shop window, it's soaking the sunlight into swathes of storm-dark cowhide and generally reassuring you that your money has been wisely invested. It feels big, too - the G05 is the broadest X5 yet - and while the dashboard is low-rise enough to put you in the mind of a luxury saloon, you still tower over it from the standard SUV vantage point. The seats are sporty to look at, but don't let that fool you: they're as soft as cream cheese and primarily designed for sinking into.
That's just fine though because it turns out the quad-turbo B57 is mostly a big fuzzy blanket made for pulling around your shoulders while you wait for your journey to finish. This, more often than not, will happen swiftly and while you're mostly staring out of the window. Which is not a criticism. In an everyday, motorway-compactor sense, the new motor is splendid and mostly as advertised. There's so much low-end gusto that the 2,275kg X5 appears to be no more burdensome to it than a horse fly is to galloping Frankel. We had a 1,615kg Cupra Ateca with us on the same day; the M50d felt miles quicker out of the blocks.
Of course 'quicker' is not quite the right word because it implies that the BMW is doing a better job of flattening your lips against your gums when in actual fact it's all about that surging, boundless accumulation of momentum which typifies most fast diesels. Naturally this impatient lunge for the horizon is best experienced between 2,000 and 4,000rpm, where the M50d delivers the kind of free-revving mid-range hurry-up that has you merging you with motorways or overtaking slower moving traffic at about a million if you're not mindful of its potential.
The B57's energy stockpile is easily generous enough to have you dipping into it manually (not least because the X5's gear-stalk still downshifts in the correct decision and with a familiar click-y precision) but truth be told, the combination of six-pot and eight-speed automatic is so overtly responsive that there's rarely a genuine need for doing so. Nor, it must be said, is there a compelling reason for combining frivolous wand-pushing with determined corner-taking. As standard, the M50d gets model-specific steel suspension with adaptive dampers and active anti-roll bars, and its bespoke setup - mated to 22-inch wheels - makes for a somewhat unflinching combination on minor UK roads.
Sure, there's not much objectively wrong in the tenacious way it changes direction, and there's no lack of reassuring heft in the steering, but the (slower) Range Rover Sport makes a better fist of rounding bends in a way that might broadly be described as 'flow'. Throw in short wave changes in gradient or camber or the abrupt thwack of poor surfacing (i.e. the idiosyncrasies of every B road everywhere) and the M50d has the intermittent habit of jostling you right at the moment when you'd prefer it to seem impassively composed.
Instead the best thing we found to do with the velvet-gloved X5 is rather what you'd imagine it's been designed for: running the length of a country as fast as you dare with as few stops as possible in between. Brim it, and the M50d reckons it'll get the best part of 600 miles from its 85-litre tank. We drove it to the Pennines and back, and it didn't put a massive wheel/tyre wrong the entire time. Granted, it only returned just over 30mpg - but try getting that from anything of comparative size with 400hp on tap.
It is that spread of performance and economy, combined with the X5's superior sense of scale, ambience, refinement and finish, that ensures there's still space in the lineup for the M50d. Had we a daily need to steamroller between Frankfurt and Munich (for example) there are precious few ways to do it better - or not while retaining a diesel SUV's usability, at any rate. In the UK that core ability is arguably of less value if you rarely venture beyond the Home Counties, although, as ever, there's a shameless sort of satisfaction in knowing that no other manufacturer's bonnet shelters a six-cylinder oil burner of quite the same pedigree. That might not be sufficient to save the M50d from extinction in the X5's next few life cycles - or alleviate the question mark over its current premium - but it makes BMW's new dinosaur a likeable one nonetheless.
SPECIFICATION - BMW X5 M50D
Engine: 2,993cc 6-cyl, quad-turbo
Transmission: 8-speed Steptronic automatic, all-wheel drive
Power (hp): 400@4,400rpm
Torque (lb ft): 561@2,000-3,000rpm
0-62mph: 5.2 seconds
Top speed: 155mph (limited)
CO2: 179g/km (WLTP combined)
MPG: 41.5 (NEDC correlated)