It is already working, not only on my gearbox but on the AUDI R8 and Lamborghini Gallardo as well.
Of course that when the oil is hot, part of it will go through te by pass, but if you make a calculation, an hole of 4 mm is 12.56 square mm. The pipe line is 78.5 square mm. This means that in hot conditions, only 1/8 of the total flow will by pass the cooler.
In order that a GL5 oil is quiet fat when it is cold, it will not easly pass into the extremely small channels which are in the cooler, contrasted by the viscosity.
More oil is going through the by pass ecause the lamination effect is very low in a 4 mm hole thenin the 1 mm channels of the cooler,
Results of a test made on CIMA T906S.
5.000 revs constant. Gearfox GFX oil in the gearbox.
Total flow at 10°C - 6.8 L/min but only 3.2 L/min are passing through the cooler ( Setrab ).
Total flow at 80°C -15.4 L/min but 11.8 are going through the cooler.
In cold conditions 47% of the oil returns directly into the gearbox but when it is sufficiently warmed up, 77% of the oil is passing into the cooler.
I do not have enough words to get into the detail for a better explanation, but believe that it works.
Ciao
Wanni