Group IV vs. Group V Base Oils

Why PAO and esters aren’t really competitors, the typical blend ratios manufacturers actually use, and where each one earns its place in the bottle. US & Canada, 2026.

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20+ years running AMSOIL across cars, trucks, and equipment, this is the question I get right after someone finally understands Group III vs. Group IV: “okay, so what’s Group V, and do I need it?”

Last updated 2026. Reviewed against published base oil chemistry and industry formulation patents.

On This Page

1. Quick answer: they’re not really competitors
2. Core differences at a glance
3. Why almost every oil blends both
4. Typical blend ratios in real products
5. When you’d actually want near-pure Group V
6. What AMSOIL uses
7. Frequently asked questions

Quick Answer

Group IV (PAO) and Group V (esters and other specialty stocks) aren’t rival base oils competing for the same job, they’re complementary. PAO is chemically inert with excellent cold-flow and thermal stability but tends to shrink rubber seals slightly. Esters are polar, cling to metal surfaces for extra film strength, and naturally swell seals back to size. That’s exactly why almost every premium synthetic motor oil blends the two rather than using either one alone.

The typical split: most PAO-based automotive oils carry roughly 10-15% ester content, mainly to solve the seal-swell problem and help dissolve additives. Racing and extreme-performance formulations sometimes push that ester content considerably higher for maximum film strength and heat resistance.

Side by Side

Core Differences at a Glance

The Real Story

Why Almost Every Oil Blends Both

PAO’s biggest practical weakness isn’t performance, it’s compatibility. Because PAO is so chemically inert, it doesn’t dissolve additive packages as readily as other base stocks, and it can cause a mild but real shrinkage in the rubber seals and gaskets throughout an engine over time.

Esters solve both problems at once. Their polarity gives additives something to dissolve into more easily, and it naturally swells seals back toward their proper size, counteracting PAO’s shrinkage tendency. This is documented directly in lubricant formulation patents: a “good PAO oil will have esters to balance out the mixture to prevent seal shrinkage,” a pattern that shows up across essentially every major PAO-based motor oil on the market, AMSOIL included.

In practice, this means asking “should I buy PAO or ester oil?” is usually the wrong framing entirely, almost every high-quality synthetic oil already contains both, working together rather than competing.

Real-World Formulation

Typical Blend Ratios in Real Products

Edge Cases

When You’d Actually Want Near-Pure Group V

For the vast majority of drivers, this is a non-question, you’re buying a PAO/ester blend either way, and the manufacturer has already made the ratio decision for you. But ester content becomes a genuinely relevant buying factor in a few specific situations:

• Track and racing applications where sustained extreme heat and the highest possible film strength matter more than cost.
• Specialty greases and hydraulic fluids, which often lean more heavily on ester or other Group V chemistry for their specific application.
• Engines with a documented history of seal issues, where the seal-conditioning properties of a higher ester content genuinely help.

Outside those cases, a well-formulated PAO/ester blend from a reputable manufacturer covers what a daily-driven vehicle actually needs.

Where AMSOIL Fits

What AMSOIL Uses

AMSOIL’s Signature Series lineup follows the same proven pattern described above: a PAO-dominant base with esters blended in for seal compatibility and additive solvency, rather than relying on Group III alone. The company has disclosed this PAO-and-ester formulation approach publicly for years, and it’s the combination behind AMSOIL’s extended-drain interval claims.

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Frequently Asked Questions

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