In BMW temperature of oil during cruising is between 110-120c. DME is running water pump and thermostat in a way that keeps oil temperature at higher level so it increases efficiency. The slower or, better word, economically you drive, the higher oil temperature is.Back in the day, we used to have this thing called the Harrier jump-jet. It could take off vertically, hover...and get this...'bow' to the crowds at airshows! Of course we had other aircraft too... Tornadoes, Jaguars, venerable Vulcans, etc but the Harrier was 'what made us great'.
Which is odd because objectively, as a fighter aircraft, it really wasn't that great. Too expensive, too complicated, too slow, too thirsty, inadequate range, too little load carrying capacity plus a marked propensity for killing its pilots in accidents but hey, those bows. Oh we clapped & cheered!
As the decades have all too quickly ticked by, I have grown to regard the HTHS test as something not unlike the Harrier; an interesting & clever technical oddity which sounds important but actually isn't.
It's technically clever because it measures fresh oil viscosity at conditions of relatively high temperature (150°C), high speed & high rotating shear (like what you get in bearings). Under these conditions, the long chain polymeric molecules that make up the oils VII can 'align' & cause a partial but TEMPORARY drop in the oil's viscosity.
To which I say, so what? First off, unless you're hammering it at Daytona, your bulk oil temperature should never get anywhere near 150°C. Typically, bulk oil temperature tracks your coolant temperature so should top out somewhere around 100°C. I'd hazard a guess that over the last five years, my car's oil hasn't even hit 100°C because moderate speed/light load driving doesn't overstress the cooling system.
Let's also talk about the relative importance of HTHS as a test. I never kept track but as a calibrated stab in the dark, over the course of a typical crankcase oil development, I'd estimate I'd run 5,000 simple, drop tube kinematic viscosities for every ONE HTHS test. Say you were doing a Euro 5W30 program. Typically, at the start of the program, you'd check if you could make a commercial finished oil with 3.5 min HTHS without falling foul of the upper KV100 limit of 12.5 max. If yes, you might then ignore HTHS until you write up the Candidate Data Package at the end of the program. That's at the development stage. When it comes to the commercial blending of oils, I doubt if HTHS ever gets checked as part of anyone's regular QC regime.
And let's also acknowledge that the importance of HTHS is regularly & systematically undermined by the same people who insist that it's important & essential! Historically, the European OEMs were fixated on the importance of having oils which have an HTHS of 3.5 min. This figured prominently in the early ACEA specs & the CCMC specs which preceded ACEA. But then A1/B1 appeared allowing 2.9 min HTHS followed by A5/B5 which demanded you meet all the wear requirements of A3/B4 but yet allowed 2.9 HTHS. And while all this is going on, the Yanks have moved en masse to 5W20 with 2.6 min HTHS, demonstrating that it is realistically do-able!
My advice is try not to get too overly fixated on HTHS. I used to be 'that person' but I'm better now...
Go to a track, DME switches into high cooling mode keeping coolant temperature at lowest possible level. However, bcs. specificities of AMerican market, many US bound BMW's do not come with same cooling gadgets as EUropean models do (oil coolers, fluid/fluid heat exchangers) and than like I did, you end up on track running oil around 150c before DME cuts power. So yeah, HTHS does matter, but depends in which aplication.