Originally posted by BL92
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Viscosity is the most important single aspect of a lubricant, so viscosity grade is very important. However, Tbone is right that you do NOT want to choose engine oil based on that alone because there are other differences that can matter.
The "TWS shears down to an xW-40" thing is many years old, based on rare occurrences, and even those cases it was often suspect (fuel dilution wasn't always ruled out). The factory oil's formulation has been updated several times since then and hasn't been shown to shear lower than xW-50 in years.
He's right that some of Red Line 5W-50's viscometrics are comparable to those of an old version of Castrol TWS. We don't know how that compares to modern formulations because not all the info is published.
He's right that one can't conclude cold start performance based on viscosity grade alone. It depends how cold you're talking about and some other obscure viscometrics.
Red Line's products aren't lightly additized. Their race oils are light on detergent additives, but we're not talking about those here (hopefully). However, it's a VERY safe bet that Castrol/BMW's additive package is better suited for street use, as Tbone says.
He's right that Castrol/BMW oil will be friendlier to cats than Red Line's (or basically anything from Penrite, from what I'm seeing), while retaining more than enough anti-wear properties that you don't have to worry about it.
The people who built this engine – the only ones who have a complete picture of what it needs – strongly advise sticking to the factory oil. This isn't like other engines, where they write a spec and say everything that meets the spec is fine; they're saying "you really should use this exact particular oil." And as BMW has gone back and back-specced more modern and thinner oils for so many of its older vehicles to simplify its supply chains, they haven't touched this one – except for that one time they released a confusingly worded bulletin and then immediately retracted it.
If you're going to insist on rolling the dice and deviating from that, at least stick to what your favorite lube manufacturer recommends. AFAICT, the only engine oil Penrite recommends for this engine is their 10 Tenths Premium 10W-60, and they actually go out of their way to say it's the only one you should choose.
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