Dubbelreducerade block kom 1970.
Intressant inlägg från Cal look forumet.
"To start with the last first. VW NEVER recommended 20w/50 on this side of the Alps. That´s something the community adapted from the racing world, where the engines were, - and are to a certain extend, built with larger tolerances so that there was room for a little expansion and less drag. In order to compensate for that and have a fatter oil film 20w/50 oil was/is widely used. In a street engine, even one with a liter output of about 70 hp/l and a 7000ish red line, (Perhaps even more than that) there is no need to build with larger tolerances to overcome these problems. 2 reasons : 1. Today we have really good oils that carries a lot more than the general oils did just 20 years ago. 2. Today we are generally much better at controlling the oil/case temps than earlier. So again, no need.
Also, in stockish engines with no external thermostat, you will soon develop a temperature problem if you run too high a viscosity, especially if nothing is done. The reason is that the oil pressure relief valve(s) act like a thermostat. IF you use a fat oil you will not notice much when driving around town. Once you get on the motorway you will notice that the oil temps climbs nice and steady, and keeps climbing to unacceptable levels, even at relatively low speeds where a stock engine should/could handle the load with no problems. (Assuming the trim and air supply is as should be) The reason is that the engine "thinks" that it is half cold, because the viscosity is high, so the relief valve is open and the system bypasses the oil instead of routing it to the cooler.
That is the very simple reason as to why 50% of all the guys that runs 20w/50 oil in their old cars have problems when going fast over longer distances.
Now, switching to a lighter viscosity will help the relief system to do its job as supposed to, and suddenly the engine does not overheat at regular freeway speeds anymore.
On engines with external thermostats and coolers this problem hardly exists because then the thermostat is temperature controlled and forces the oil through the cooler when it gets warm enough.
Then there is the drag of fatter oil and big oil pumps. I did some testing on a 2 l. type 4 some years ago, where I tried using a 30 mm pump with 20w/50 Castrol (deliberately chose that one as that is what people generally think is good) also a 26 mm pump. and the same pumps with 15w/40 Shell Rimula with a shot of ZDDPlus. The difference was even "worse" than I expected on this low revving engine with peak hp at 5000 rpm
Note, this engine has otherwise stock oil system.
30 mm pump and Castrol 20w/50, 101 hp @ 5050 and 170 Nm peak torque @ 3540 rpm.
26 mm pump and Castrol 20W/50, 103 hp @ 5100 and 173 Nm peak torque @ 3600 rpm.
30 mm pump and Rimula 15w/40, 104 hp @ 5100 and 174 Nm peak torque @ 3600 rpm.
26 mm pump and Rimula 15w/40, 107 hp @ 5170 and 177 Nm peak torque @ 3650 rpm.
As you can see there is a significant difference depending on which set up is used. A 6 hp difference in drag at 5000 rpm is A LOT. Think of how much the oil pressure relief system has to work extra to compensate in the "worst" set up contra the "softest" set up. this engine resided in a buggy with huge rear tyres. But even with those the power gain was definitely noticeable when you drove the car.
Now, wrt the dual relief question.
I do not buy the "heating up" answer throught the case to be a motive for altering the oil system to dual relief. The reason is that the oil is warm already, so to speak. It either comes directly from the sump/pump or it comes from the cooler, and it happens so fast that there is very little time to pick up temperatures from the case before it reaches the bearings or the lifters and subsequently the cylinderheads.
Still IMO, it is most likely not the total answer, the main reason is that with the dual relief system you now have larger oil passages and a larger oil pump. This ensures superior oil supply to the bearings at all times. But the pressure/volume is actually too large when the engine is cold and also at higher rpm. When the engine/oil is cold the main bleed relief is at the pressure plunger at the pulley, as it is adjusting the amount of oil going to the cooler. The resistance is high, so the oil gets bypassed directly into the oil gallery and excessive pressure get routed back into the sump by the front relief plunger and also the pressure relief in the back of the case.
when the oil gets warm the rear relief plunger closes the bypass and sends the oil through the cooler and then down into the oil gallery. At this stage the volume is relatively high because the oil floats well since it is warm. Now the volume is higher than what is actually needed for the bearings etc so now the front plunger bleeds the excessive volume back to the sump.
The WBX does not need the second bleed relief as much because it has a heat exchanger, which means that the oil reaches operating temperature fast (As soon a the water temp is up the oil temp is too) Also the WBX and most of the type 4 cases have a slightly different oil system. They bleed excessive oil right back to the oil pick up, which again aids in supply stability, but also aids in quicker oil warm up Also the supply to the lifters are larger, which means less resistance so the difference in oil pressure from the main gallery to the outer lifter is minimized. - This is also why we perform the HVX mods to engines that are high rpm, high power, and especially those which run sustained high rpm to ensure adequate lubrication at all times"