Originally Posted By: MrWideTires
Looking at Castrol's oil specs for curiosity:
http://www.castrol.com/liveassets/bp_internet/castrol/castrol_usa/STAGING/local_assets/downloads/p,q/pds_syntec_usa.pdf
I see that their 10w40 is THINNER than their 5w40???? what gives???
and the 5w50 actually seems like a pretty good oil, anyone used it yet??
There's no big mystery here. Remember, the SAE viscosities represent RANGES of viscosity, not an absolute thickness that's the same for all 30s, 40s, etc. Beyond that, the "w" figure is defined differently, and has little, if anything, to do with the hot viscosity.
You could also, for example, compare GC (german castrol, a long-time favorite here) 0w-30, with Mobil 1 (standard flavor) 10w30. The 0w30 GC is waaaaaay thicker when hot than is the Mobil 1 10w30. GC is 12.5 cSt at 100C, whereas the Mobil 1 10w30 is "only" 10.0 cSt at the same hot temp.
The SAE rating scheme is, in my opinion, hopelessly impervious to understanding by normal consumers, and should be scrapped at once. Understanding that this will not happen any time soon, you just have to do the homework. A critical, early lesson to absorb is that, for the most part, you MUST NOT try to tie the "w" (cold) rating to the hot rating. They measure different qualities of the oil, and trying to link the two will only lead you off the path into a quagmire of confusion.
