OCP Viscosity Index Improver: Grades, SSI, and How to Choose

An OCP viscosity index improver is an ethylene-propylene copolymer (OCP) engineered to raise the viscosity index of a lubricant — the polymer that turns a straight-grade base oil into a 5W-30 or 15W-40 multigrade. OCP is the dominant viscosity index improver chemistry worldwide because it balances thickening efficiency, shear stability, and cost better than any alternative at production scale. For blenders, the practical question is not "OCP or not" but "which OCP grade" — and the answer lives in the shear stability index (SSI) and thickening efficiency of the specific product.

What Makes an OCP Viscosity Index Improver Different

An OCP viscosity index improver is a random copolymer of ethylene and propylene with a saturated, flexible backbone. Three design parameters separate commercial grades:

1. Ethylene/propylene ratio — sets solubility and low-temperature behavior. Grades are engineered amorphous to avoid the wax-like crystallinity that high ethylene content can introduce.
2. Molecular weight — the primary lever on thickening efficiency: longer chains thicken more per gram.
3. Molecular weight distribution — the primary lever on shear stability: a narrow distribution removes the long-chain tail that breaks first under shear, delivering lower SSI at the same average molecular weight.

Because these parameters are controlled during polymerization, two "OCP" products from different suppliers can differ substantially in real performance. Buying on declared SSI, thickening efficiency, and molecular weight data — not on the product name — is the correct practice.

SSI: The Property That Decides the Application

The shear stability index measures the percentage of thickening effect permanently lost under mechanical shear, by the Bosch injector method (ASTM D6278, 30 or 90 cycles) or sonic shear (ASTM D5621). For an OCP viscosity index improver, SSI is the application filter:

• SSI ≤ 20–25: heavy-duty diesel oils, long-drain passenger-car oils, and gear applications where grade retention is critical.
• SSI 25–35: the mainstream engine oil range — API SP, ILSAC GF-6 passenger-car oils with normal drain intervals.
• SSI 35–45: economy oils and non-critical industrial lubricants where cost dominates and grade drift is tolerable.

A lower-SSI grade costs more per unit of thickening, so the SSI selection should match the real shear severity of the application — overspecifying SSI wastes money, underspecifying it produces oils that shear out of grade.

T613 vs T614: Two Workhorse OCP Grades

Minglan Chemical supplies two OCP viscosity index improver grades that cover the mainstream formulation space:

• T613 — a standard OCP grade balancing thickening efficiency and shear stability, suited to passenger-car and light-duty engine oils, hydraulic fluids, and general industrial multigrades.
• T614 — a lower-SSI OCP grade for formulations where grade retention matters more: heavy-duty diesel oils, long-drain passenger-car oils, and applications with severe shear environments.

Both grades are supplied as diluent-cut concentrates with declared active content, KV100, SSI (with test method), and molecular weight data on the certificate of analysis, so treat-rate calibration and lot-to-lot verification are straightforward.

How to Select and Dose an OCP Viscosity Index Improver

1. Fix the specification: identify the SAE J300 grade window and the application's shear severity.
2. Select the SSI class: heavy-duty → T614 class; mainstream passenger-car → T613 class.
3. Calibrate the treat rate: blend 2–3 concentrations (0.3/0.6/0.9 wt% active) in your actual base oil, plot KV100, and interpolate the dose that centers the window.
4. Verify on the full package: CCS (ASTM D5293), MRV (ASTM D4684), HTHS (ASTM D4683), and post-shear grade retention (ASTM D6278) with the final detergent/dispersant/antiwear combination.

Because thickening efficiency shifts with base oil solvency, always recalibrate per base oil — do not carry treat rates across projects.

FAQ

What does OCP stand for? Olefin copolymer — a copolymer of ethylene and propylene used as a viscosity index improver in lubricants.

What is the difference between T613 and T614? T613 is the standard OCP grade for mainstream engine and industrial oils; T614 is the lower-SSI grade for heavy-duty diesel, long-drain, and severe-shear applications where grade retention is critical.

How much OCP viscosity index improver is in engine oil? Typically 0.2–1.0 wt% of active polymer for passenger-car oils, varying with grade span, base oil, and the product's thickening efficiency.

How do I verify OCP quality on receipt? Check the CoA for SSI (with test method), KV100, active content, and molecular weight data; run an incoming KV100 check on the concentrate as a fast consistency proxy.

Conclusion

An OCP viscosity index improver is the workhorse of multigrade formulation — high thickening efficiency, tuneable shear stability, and low cost per unit of viscosity. The selection is made on SSI and molecular weight distribution, matched to the shear severity of the application, and verified on the full package. Minglan Chemical's T613 and T614 OCP viscosity index improver grades cover the mainstream and heavy-duty formulation space with full lot documentation — contact us for data sheets, samples, and treat-rate support. For how the polymer behaves as a viscosity modifier in finished oils, see our OCP viscosity modifier guide.