Originally Posted by 1JZ_E46
Well, technically a 40 grade can be lower than that, but I've never seen it. Most of the light 40 grades are 3.6-3.7 HTHS.
Nope. ACEA A5/B5 and SAE xW-40 are mutually exclusive. According to SAE J300 an xW-40 grade must be >3.5 cP (for x=0, 5 & 10) or >3.7 cP (for x=15, 20 & 25).
Originally Posted by 1JZ_E46
A3/B4 and A5/B5 must pass the exact same wear, deposit, oxidation, etc tests.
Not quite. Yes, it must pass the same tests but A5/B5 has tighter limits on oxidation with biodiesel (section 1.14), engine wear (OM646LA, section 2.7) and piston cleanliness (VW TDi, section 2.8).
A5/B5 is arguably a tougher spec than A3/B4 because it must be as good as or better than A3/B4 whilst also having lower HTHS and delivering a fuel economy benefit.