AMSOIL vs Royal Purple
Royal Purple has built a strong brand around performance marketing. Here’s how the deposit-control data actually holds up, plus a piece of the brand’s own advertising history worth knowing before you decide.
Alan Williams · AMSOIL Direct Jobber #1243776 · Tomball, TX · 225-441-6397
Deposit Control — TEOST 33C (ASTM D6335)
The Test That Actually Matters for Turbo Engines
The TEOST 33C test (ASTM D6335) measures how many milligrams of deposit a motor oil leaves behind under simulated turbocharger heat — the industry-standard bench test required by the API SN Resource Conserving specification. Independent third-party lab testing put both AMSOIL Signature Series 5W-30 and Royal Purple 5W-30 through this exact test.
7 mg
25.4 mg
Lower mg = fewer deposits = cleaner engine.
AMSOIL produces 3.6x fewer high-temperature deposits than Royal Purple in this test. For turbocharged engines specifically, this matters more than most spec-sheet numbers: turbochargers spin at 100,000+ RPM and stay hot for minutes after shutdown, and it’s exactly that heat-soak period where deposit-forming oxidation happens fastest.
Advertising Claim History
Worth Knowing Before You Read the Marketing
In 2009, the National Advertising Division (NAD) of the Council of Better Business Bureaus reviewed a challenge brought against Royal Purple by a competing lubricant manufacturer. NAD examined a series of Royal Purple’s performance claims — including “increases horsepower and torque by as much as 3%,” “reduces engine wear by 80%,” and “provides film strength up to 400%” — and recommended Royal Purple modify or discontinue several of them, citing insufficient independent evidence to support the specific figures advertised.
This isn’t AMSOIL’s opinion or mine — it’s a matter of public advertising-industry record. I’m including it here because it’s relevant context for any brand claim you read, from any manufacturer, including AMSOIL: ask what test backs the number, and whether that test is named and independently verifiable. Every figure on this page cites the specific ASTM test behind it for exactly that reason.
Where Royal Purple Is a Reasonable Choice
Intellectual Honesty Matters
Royal Purple is a legitimate synthetic oil manufacturer, not a bad product. It’s a reasonable choice for:
- Drivers who buy it locally at a parts store and change oil on a short, standard interval where extended-drain chemistry matters less
- Budget-conscious buyers who won’t have access to AMSOIL’s Preferred Customer wholesale pricing
- Anyone whose priority is a name-brand synthetic on the shelf today rather than ordering ahead
Quick Comparison
| Metric | AMSOIL Signature Series | Royal Purple |
|---|---|---|
| Deposit control (TEOST 33C) | 7 mg | 25.4 mg |
| Rated drain interval | 25,000 mi / 1 yr | 12,000 mi (normal & severe service) |
| Available wholesale pricing | Yes — 25% off, Preferred Customer | Retail parts-store pricing |
Common Questions
Is Royal Purple a bad oil?
No — it’s a legitimate full synthetic oil. The data above shows AMSOIL outperforming it on deposit control specifically, not that Royal Purple fails to protect an engine under normal use.
Can I switch from Royal Purple to AMSOIL without a flush?
Yes. AMSOIL is compatible with all synthetic and conventional oils. Drain your existing Royal Purple at your normal interval and refill with the correct AMSOIL Signature Series viscosity grade — no flush required for a clean switch.
What does the 2009 NAD ruling actually mean for me as a buyer?
It means some of Royal Purple’s specific marketing numbers weren’t backed by independent evidence NAD considered sufficient at the time. It’s not a safety finding — it’s a reason to ask any brand, AMSOIL included, what test stands behind a number before taking it at face value.
Does switching to AMSOIL void my warranty?
No. Under the Magnuson-Moss Warranty Act, a manufacturer cannot void your warranty simply because you used a different oil brand, provided it meets the required API specification and viscosity grade.
Try AMSOIL at 25% Off
See the deposit-control difference in your own engine. No obligation to sell anything — just the same oil I run myself.
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AMSOIL and Royal Purple are registered trademarks of their respective owners. Lube Oil Sales is an Authorized AMSOIL Independent Dealer (Dealer #1243776). This page contains affiliate links. TEOST 33C data per ASTM D6335, from an independent third-party lab test commissioned by AMSOIL. 2009 NAD advertising review is a matter of public record with the Council of Better Business Bureaus. Last updated: July 2026.