So my Detroit DD15 came factory filled with an FA-4 spec oil, but nobody seems to know (or want to disclose) what brand or viscosity. I suspect it's a 5 or 10W-30 since the whole point of FA-4 was to coax fuel economy from a thinner oil with a much higher shear temperature. Unfortunately, any FA-4 oil seems to still be unobtanium on the retail market. I can purchase Rotella T5 Ultra 10W-30 FA-4 in 55 gallon drums @ $26/gal, but that price makes it economically unfeasible (the DD15 takes 45 qts per oil change). The added cost of FA-4 vs the same CK-4 spec Rotella (@ $14.50/gal) will never be recovered in the (theoretical) fuel savings, so I'm sticking to CK-4 for now until it becomes more readily available and the price drops. My question to the community is what, if any experience have you had with any FA-4 spec oil? Brands, viscosity, price and fuel economy?