General question: viscosity vs quantity of additives

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Sep 6, 2003
Loveland, Ohio
Years ago, when the car manufacturers started specifying 10W30 over 10W40, they claimed it was because of the number of engines they found sludged up, and it was because 10W40 had over 25% additives. Maybe this was just a cover to get lower viscosity hours adopted to get better gas mileage? It seems to me that 5W30 oils would also require more additives (VI Improvers, whatever) to cover the range. Now I see where many here on this site recommend 15W50 or some other higher range of vis, and I would think that would also require quite a bit of additives to do this? Does anyone else share my concerns - or can someone educate me on this subject?
I take it your refering to VII's, or viscosity index improvers. Your right though...the wider the spread, the higher amount of VII's the oil will need to get the job done. VII's are far superior today compared to the "black death/sludge" days of old, so unless you run a good modern 5w-30 mineral oil a long time, you will not have a big buildup. Synthetics are a completely different story...they don't have to use nearly the amount of VII's are mineral oils, so, of course, they are much more shear stable. Obviously, the closer the spread, the better, in terms of lower VII levels. Others can greatly expand on this...I'm just too tired!
Some of the oils, especially the 10W40's you speak of, suffered the "Black Death" syndrome because of highly shear unstable Viscosity Index Improvers, which were added at or above the 10% by weight category. These unstable VII's resulted in terrible sludging. The older oils (SC to SE) of the late 50's and early '60's had bases of Group I oils which were high in sulfur content as well. Most of today's modern dino oils contain the better shear stable VII's and at lower treatment rates (approx. 7% or lower). A 10W50 dino oil would certainly contain a lot of VII's. Full synthetics of course would contain a lot less (approx. 3%). Most of today's daily drivers can tolerate oils between approx. 4.0 cSt and 13.5 cSt without major wear problems.
One could 1. add about 8% of a PMA-D ester (polymethacrylate dispersant) VII, or 2. Use the PMA-D at 3% in a binary compound of GIII, and petroleum Brightstock, or 3. Use the PMA-D at 5% in ternary compound with GIII, a little less Brightstock and some heavier paraffinic mineral oil.
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