Choose between synthetic and conventional engine oil after matching the vehicle’s viscosity, API or ACEA category, and manufacturer specification. Base-oil marketing cannot make an incompatible product correct, and “full synthetic” does not create a universal drain interval.
If the manual requires synthetic oil or only suitable products are synthetic, use it. If both types are permitted and both carry the complete requirement, compare current product data, operating conditions, availability, and price.
Watch: Synthetic vs. Conventional Oil
Editorial correction: the video's teardown, “four times wear,” turbo repair price, volatility ranges, and blend-percentage claims are not supported as universal comparisons. Use the product-level and vehicle-specific method below.
The direct comparison
| Question | What the label can tell you | What still needs evidence |
|---|---|---|
| Is it synthetic or conventional? | The marketer’s product type | Exact base-stock composition may not be disclosed |
| Is it the right viscosity? | The SAE grade on the label | Whether that grade is permitted for your vehicle |
| Does it meet a service category? | API mark or stated ACEA claim | Exact category, market, and current licensing or claim status |
| Does it meet an OEM requirement? | Named approval or recommendation wording | Whether the exact product/grade is formally approved and current |
| Will it last longer? | Nothing universal | Vehicle schedule, operating conditions, and product-level test data |
The best buying order is compatibility first, then comparative performance.
What “synthetic” does not prove
The word alone does not prove a specific evaporation loss, cold-cranking result, deposit performance, fuel-economy improvement, or service interval. Those depend on the exact formulation and test result.
ASTM D5800, for example, defines standard methods for measuring evaporation loss. It does not state that every conventional oil falls within one range or every synthetic within another. Use the exact product’s current technical data sheet when comparing volatility, and make sure the test method is the same.
The same caution applies to base-oil groups. A consumer label usually does not disclose enough information to reconstruct the complete formulation, and additive chemistry also affects finished-oil performance.
Heat, cold, and turbochargers
Heat and cold are legitimate reasons to pay attention to tested performance, but they do not create universal rules. Compare the permitted SAE grades in the vehicle manual, then look at relevant product data such as cold-cranking, pour point, viscosity index, or evaporation loss where published.
For a turbocharged engine, use the exact oil specification required by the manufacturer. Do not infer that every turbo requires one base-oil type, one viscosity, or one interval. If a temperature or oil-pressure warning appears, synthetic oil is not a repair; follow the vehicle’s shutdown and diagnostic procedure.
How to compare two compatible products
Once both products match the complete vehicle requirement:
- Confirm the exact product name and grade—not just the brand family.
- Compare current approvals or licensing.
- Use comparable test methods from current technical data sheets.
- Check bottle size, local price, and availability without assuming price equals protection.
- Keep the manufacturer service interval unless qualified vehicle-specific guidance says otherwise.
Shell’s own product pages illustrate the proper scope: the company describes performance attributes for its named semi-synthetic and full-synthetic formulations. Those statements support those products, not a universal number for every oil in the category.
Frequently Asked Questions
Does my car need full synthetic?
Use the owner’s manual. If it requires synthetic, follow it. If it specifies only performance requirements, choose a product that satisfies every listed requirement and any formal approval wording.
Can synthetic oil extend my change interval?
Not by itself. Follow the vehicle’s maintenance schedule or oil-life system. An extended-drain marketing claim does not override the manual.
Can I switch from conventional to synthetic at high mileage?
Do not use a universal mileage threshold. First check for active leaks, abnormal consumption, and the permitted specification; then follow manufacturer or qualified service guidance for that engine.
Is synthetic blend a fixed percentage?
Do not infer a universal percentage from the front label. Evaluate the exact finished product’s specifications and approvals.
Is expensive oil always better?
No. An expensive incompatible oil is still wrong. Among compatible products, compare relevant product data and total cost rather than brand prestige alone.
Sources
- American Petroleum Institute, Motor Oil Guide, API service categories, certification marks, and consumer selection guidance. Accessed July 27, 2026.
- SAE International, SAE J300_202405 Engine Oil Viscosity Classification, Scope, viscosity-classification purpose and limits. Revised May 9, 2024; accessed July 27, 2026.
- ASTM International, ASTM D5800-21, Scope, standard test methods for evaporation loss of lubricating oils. Accessed July 27, 2026.
- Shell, Shell Helix Semi-Synthetic Motor Oils, product-specific claims for the named Shell formulations. Accessed July 27, 2026.
- Shell, Shell Helix Ultra, product-specific claims for the named Shell full-synthetic formulations. Accessed July 27, 2026.
Read Next
- Best Engine Oil: Four Checks That Matter
- Top Engine Oil Brands: Product-Level Comparison
- 5W-30 vs. 5W-40 Engine Oil
| *Last Updated: July 2026 | 7-minute read* |