Thickeners...or thinners?

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Sep 20, 2003
AB, Canada
Someone in another thread is "debating" whether 5W30 is thin oil with thickeners; or a thick oil with thinners...?

I was always under the assumption that most oils start off as a rather thin base oil, using viscosity modifiers to get the desired operating temp. viscosity.

...And I was always told to use the narrowest viscosity spread possible, and use straight weights if conditions permit ... to high a spread (IE 5W50, 10W60) and the modifiers would break, or shear down, causing the oil to thin out, oxidize and leave deposits.

Is this always the case; what about synthetics; how do the 0Wxx's work; why does a 15W50 M1 seem more stable than a a 15W40 the most stable oil, or maybe the 5W20's...?

Someone will correct me if I'm wrong, but I've always assumed that with a good 5w30 synthetic, it starts out with a thicker base oil and in a lot of cases it naturally has a good cold cranking ability, so it doesn't often need much in the way of "doctoring up" in order to get it to qualify for 5w status.

But with a dino 5w30, it starts out with a thinner base oil and then gets a lot of viscosity index improvers in order to get the desired 100c viscosity. This is why a 5w30 dino will thin out into a 5w20 so easily.
This smells like the urban legend of "paraffin" oils needing lots of help thinning out to an appropriate liquid.
As I understand it, they all start out thin. Liquids, even water, naturally thin out as they warm. The straight chain alkanes, abundant in the parafin base oil stocks, thin out about the fastest. Straight 30 weight is very thin at operating temperatures. The PAO's and esters in the synthetics, naturally thin out slower. Poorly soluble polymers become more soluble, contributing more viscosity, offsetting the loss of viscosity of the base oil. The parafins that thin out the quickest are otherwise very stable, more so than PAO's and esters. However the polymers added to stabilize the viscosity are much less stable, being the first to shear down. A straight weight oil might be a economical choice for a engine that sees few cold starts and little temperature fluctuation. In this millennium, with improved additives, the average engine is best off with a moderate multigrade. I have no problem with doing as GM says, and running 5W-30 in my 02 Cavalier. Likely the cheapest stuff I could find with an SL rating is better than the 10W-40 Valvoline I was using in the mid 70's. I would stick close to what your owners manual says.
A good general assumption for dino oil is that the front (w) number is roughly the base oil and the polymers are addeded to reduce thinning to get the back number. Now there may be some leeway with pour point depressants to use a thicker base oil and still qualify for a lower front number, but I am not sure. Synthetics need far less polymers to get the same grade range as a dino and in some cases a straight weight synthetic will have the performance to actually qualify as a multigrade with no polymers added. Interestingly, and apparently because of the polymers, two oils a straight weight and a multigrade with the same 100C viscosity, will actually show a higher viscosity for the multigrade at 120C. So a straight weight is great but may offer less protection than a multigrade in a hot running engine (assuming the polymers don't sheer or cook).

Originally posted by geeeman:
I was always under the assumption that most oils start off as a rather thin base oil, using viscosity modifiers to get the desired operating temp. viscosity.
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It depends on the BASE STOCK... The cheap and the lower quality oils tend in general to start with a thicker oil as a base, the better ones at least the ones we use are thin. The cheaper ones will start with a "150 brightstock" say for example this being a group I oil and the quality or super quality dinos can/or will start with a much better base oil.
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