Straight 30 is 3.4-3.6
Originally Posted by Silk
So where would a straight 30 sit, see all info on multigrades of course, but they gloss over the straight weights. Curious because I am using it in my BMW motorcycle this year, and BMW recommend it for a bit narrower range than the normal 20W-50 everyone uses. Up to 30c...and we've been going a tad over that lately, sump temps are good with a temp gun. They must be happy with it's HTHS, and the bike is running great.
Straight 30 is around 3.4-3.6.
Straight 20s were around 2.9.
What happened was that multigrades came in, and were widely adopted. But over time, they realised that the engines were not being protected as well as the "grade" predicted that it should have, given the grade.
So they started looking at what happened, when viscosity modifying polymers were exposed to high shear rates in engines, and found that the polymers stretched out, and lost (part of) their thickening abilities, behaving as a thinner oil in the bearings.
That's when HTHS was developed, and became a part of the J300 oil specs.
30 grades, which were typically HTHS 3.5 were given a 2.9 minimum (where a straight 20 grade would be). 20s were given 2.6. 40s were given 3.5 for 15W, 20W, and 25W 40s, while the 0W, 5W, and 10W 40s were given 2.9 minimum same as the 30s.
Below is a test that I think was really clever...a Pommie 4 cylinder that they tapped into number 4 main bearing, and supplied with a constant pressure oil feed, measuring it's flow rate to infer viscosity.
It shows how a mono is a Newtonian fluid, it's viscosity doesn't get affected by shear rate, and how multis have a Newtonian early stage, tehn change viscosity rapidly over RPM ranges, then establish the "second Newtonian" range where the stretched out polymers don't "shear" anymore..
Personally, I prefer to pick the second stage (HTHS), and not have it overly "thick" in the early stage...that's whiy I'm not a fan of polymeric VIIs...espcaially in temperate climates.