I know that the use of AI here is frowned upon, but I didn't know what this meant, so I Grokked it, and I think others may find this useful, too. Per Grok:
"**"Full shear" conditions**, as used by Gokhan (a well-known member on BITOG/BobIsTheOilGuy forums), refers to engine lubrication scenarios where **viscosity index improvers (VIIs or VMs — polymeric viscosity modifiers)** in multigrade motor oils are effectively **temporarily sheared out** or excluded from contributing to the oil's viscosity.
Under normal conditions, HTHS (High-Temperature High-Shear) viscosity — measured at around 10⁶ s⁻¹ shear rate — still includes some partial contribution from those long-chain VII polymers, which temporarily align and thicken the oil under moderate-to-high shear.
In "full shear" (or **high-temperature, full-shear / HTFS** conditions), the shear is so extreme — or the geometry so tight — that the VII molecules are completely stripped away or unable to provide any thickening effect. The oil's effective viscosity then drops to essentially that of the **base oil** (plus the additive package, but excluding the VII contribution). This is sometimes called the "second Newtonian" region in rheology.
Gokhan popularized (and likely coined for BITOG discussions) the term **HTFSV** (High-Temperature Full-Shear Viscosity) and built calculators/spreadsheets to estimate it for commercial oils based on their KV100, HTHS, and other specs. He argued that this **HTFS/base-oil viscosity** at ~150°C is often more relevant to actual wear protection in critical engine areas (tight clearances like bearings, timing chains, valvetrain) than the standard HTHS number alone — especially in modern engines with very high shear rates or narrow passages where VIIs can't "fit" or function.
The specific quote you mentioned —
> "A demonstration of what Gokhan had been calling "full shear" conditions."
— comes from a September 2025 BITOG thread about a study on timing chain wear. In that context, the poster was pointing out that the study showed thicker **base oils** reduced chain elongation/wear far better than relying on VII-thickened oils. The explanation: in the extremely tight clearances of a timing chain, the large VII polymer coils are physically excluded (they can't squeeze into the microscopic gaps), leaving only the base oil to form the lubricating film — exactly mimicking "full shear" behavior even without ultra-high mechanical shear rates or extreme temperatures. One reply noted: "stripping the VM out acts the same as high shear."
In short, it's a way to describe situations (very high shear, extremely tight clearances, or both) where the oil behaves as if it's **monograde / base-oil only** — no temporary VII thickening remains — and that's often the regime that determines real-world wear in certain engine components. Gokhan has long advocated for oils with lower VII content and higher estimated HTFS for better durability in those conditions."
I agree!
