Engine Oil: Synthetic vs Semi-Synthetic vs Mineral — How to Choose by API SP / ILSAC GF-6 / JASO MA Standards

Selecting engine oil for a truck fleet, passenger vehicles, or industrial machinery in Thailand in 2026 is no longer simply a matter of choosing "premium grade" or "lowest cost." Since API officially launched Service Category SP on 1 May 2020 and ILSAC released GF-6A/GF-6B, Turbo GDI engines — which are fitted across virtually every modern vehicle brand — require oils that pass LSPI protection and timing chain wear testing. Older SN/SM-spec oils do not pass these tests. This article is an engineering summary for procurement and maintenance managers who purchase oil by the pallet.

Base Oil Groups — The Root Difference Between Mineral, Semi, and Full Synthetic

API 1509 Annex E classifies base oils into five groups, using Saturates content, Sulfur content, and Viscosity Index (VI) as the criteria:

• Group I (Mineral, solvent-refined) — VI 80–120, Sulfur > 0.03%. Lowest cost. Used in industrial gear oil, mineral monograde, and older machinery.
• Group II (Mineral, hydrocracked) — VI 80–120, Sulfur < 0.03%. The primary base for modern Semi-Synthetic engine oils.
• Group III (Mineral, severely hydrocracked / "Hydroisomerized") — VI > 120, Sulfur < 0.03%. The base for most "Full Synthetic" oils on the market (permitted by market convention to be labelled "synthetic").
• Group IV (PAO — Polyalphaolefins) — 100% synthesised in a chemical plant. Lowest pour point and highest VI in the non-ester group.
• Group V (Esters, PAG, etc.) — Used as blend additives to enhance performance and additive solubility.

Semi-Synthetic = a blend of Group I/II + Group III/IV (typically 10–30% Group IV). This is a marketing term with no minimum percentage defined by API, so the PAO content of Semi-Synthetic products varies widely between brands, from 5% to 40%.

API SP + Resource Conserving — The Specification for Modern Turbo GDI Engines

API Service Category SP took effect on 1 May 2020, replacing SN/SN Plus. It added seven key tests — the most important for engines used in Thailand are:

1. Sequence IX (LSPI test) — Protects against Low-Speed Pre-Ignition in Turbo GDI engines (all modern passenger cars using 1.0L–1.5L turbocharged engines).
2. Sequence X (Chain wear test) — Prevents timing chain stretch in direct injection engines.
3. Sequence VIE/VIF (Fuel economy) — Meets the ILSAC GF-6 fuel economy targets.
4. Supports SAE 0W-16 for ILSAC GF-6B (Toyota Dynamic Force, Mazda SkyActiv-X engines).

An oil bearing the API Service Symbol "Donut" SP + Resource Conserving label meets both wear protection and fuel economy requirements.

ILSAC GF-6A vs GF-6B — Choosing by Viscosity Grade

ILSAC GF-6 is divided into two sub-grades:

• GF-6A — SAE 0W-20, 5W-20, 5W-30, 10W-30. Backward compatible with GF-5 (interchangeable).
• GF-6B — SAE 0W-16 (and lower). Not backward compatible. Has a lower HTHS requirement for increased fuel economy, but should only be used in engines where the manufacturer specifies it.

The heart of ILSAC GF-6 is the Sequence VIE/VIF test — Phase 1 fuel economy improvement ≥ 2.3% and Phase 2 (sustained) ≥ 1.2%, both measured against a baseline SAE 20W-30 mineral oil.

JASO MA / MA2 / MB — For Motorcycle Wet Clutches

JASO T903 classifies motorcycle oils according to the JASO T904 Clutch Friction Test, measuring three values: Dynamic Friction Index (DFI), Static Friction Index (SFI), and Stop Time Index (STI):

• JASO MA — For general wet clutch motorcycles (no friction modifiers).
• JASO MA2 — For wet clutch motorcycles with catalytic converters (most motorcycles from ~2010 onward). Higher friction performance than MA.
• JASO MB — For scooters with CVT transmissions. Contains friction modifiers (similar to automotive oil). Must not be used in wet clutch motorcycles — will cause clutch slip.

Comparison Table — Mineral / Semi-Synthetic / Full Synthetic

Decision Tree — How to Choose for a Thai Fleet

flowchart TD
  Start[Start: What type of vehicle / machinery?] --> Type{Type}
  Type -->|Turbo GDI passenger car
2018 onwards| TGDI[Must use API SP + ILSAC GF-6A/B
HTHS ≥ 2.9 cP, LSPI tested]
  Type -->|Diesel truck / pickup
older, pre-2018| OldDiesel{Engine age / mileage?}
  Type -->|Naturally aspirated petrol
pre-2018| OldGas{Annual mileage?}
  Type -->|Motorcycle wet clutch| MC[JASO MA or MA2
No resource conserving oils]
  Type -->|Scooter CVT| Scoot[JASO MB
Resource conserving formulas OK]
  Type -->|Forklift / generator
industrial| Indus[Mineral CF/CI-4
15W-40 per OEM manual]

  TGDI --> Synth1[Full Synthetic 0W-20 or 5W-30
Drain every 10,000–12,000 km
Adjust for Severe Service in Thailand]
  OldDiesel -->|< 200,000 km| Semi1[Semi-Synthetic 10W-40
API CK-4 or CJ-4
Drain at 7,500–10,000 km]
  OldDiesel -->|> 200,000 km| Min1[Mineral 15W-40
API CI-4
Drain at 5,000–7,500 km]
  OldGas -->|> 30,000 km/year| Synth2[Full Synthetic
SAE 5W-30 or 5W-40
Payback within 12 months]
  OldGas -->|< 30,000 km/year| Semi2[Semi-Synthetic 10W-40
API SN/SP
Drain at 7,500 km]
  MC --> MCgrade[10W-40 or 20W-50
JASO MA2]
  Scoot --> ScootGrade[10W-30 or 10W-40
JASO MB]
  Indus --> IndusGrade[Per engine hours
Oil analysis every 250 hours]

Figures in the diagram are based on Severe Service conditions in Thailand (traffic congestion, ambient temperature 28–38°C, short trip cycles).

Practical Procurement Recommendations

1. Always check the PDS (Product Data Sheet) before ordering. Review the actual numbers — VI, HTHS, NOACK, TBN — not just the "Full Synthetic" label.
2. Compare HTHS values. Turbo GDI engines require ≥ 2.9 cP; heavy-duty diesel engines require ≥ 3.5 cP. Oil with HTHS below these thresholds accelerates bearing wear.
3. Check NOACK Volatility. Lower values mean lower oil consumption and fewer deposits. Below 13% is considered good; below 10% is excellent.
4. Compare Total Cost per km, not price per litre. Fleets that track fuel economy before and after an oil change consistently find that Full Synthetic pays back through fuel savings plus extended drain intervals within 8–12 months.
5. Collect Used Oil Analysis samples. Take a sample every 5,000 km the first time you change brand. Monitor TBN trend (should retain ≥ 50% of initial value), viscosity drift (should not shift ±10%), soot, and wear metals (Cu, Fe, Pb).
6. For large fleets (> 50 vehicles), negotiate bulk pricing on 18L pails or 200L drums. This typically saves 15–25% compared with buying 4L retail bottles.
7. Do not mix different API spec oils. Blending API SP with SN/SM oil reduces LSPI protection.

Summary for Thai Factories and Fleets

For any vehicle purchased from 2020 onward, API SP + ILSAC GF-6A is the minimum required specification — not optional — particularly for Turbo GDI vehicles. For older, pre-2018 NA engines, Semi-Synthetic API SN/SP is cost-effective and extends engine life. Industrial machinery such as gensets, forklifts, and compressors can continue using Mineral 15W-40 per the OEM manual, with oil analysis performed every 250 operating hours to track degradation trends.

The Sahawatthanakit (1988) Engineering Team maintains a cross-reference table for fleets running multiple grades within the same category, including SK ZIC X-9000 (Full Synthetic, API SP, ILSAC GF-6A) distributed through the Oranoss channel. Request a quotation along with the PDS and spec sheet before placing any bulk order.

Frequently Asked Questions

Can I use Full Synthetic in an older vehicle (10+ years)? I'm worried about leaks. Yes — and in most cases it is actually safer. The belief that "Synthetic breaks seals" dates back to PAO Group IV oils from the 1970s–80s, which were ester-rich and attacked older NBR rubber seals. Modern synthetics use Group III/IV base oils that fully comply with Seal Compatibility requirements under API SP/ILSAC GF-6. The key caveat: if an old engine already has hardened seals that are about to leak, the cleaner synthetic may dissolve the sludge deposits that were plugging the gaps — making it appear as though the synthetic caused the leak. The safe approach is to start with Semi-Synthetic 10W-40 first, then upgrade to Full Synthetic at the next service interval.

The 10,000–15,000 km drain interval quoted for Full Synthetic — is it realistic for Bangkok driving? The figures stated by vehicle manufacturers are for Normal Service. Driving in Bangkok qualifies as Severe Service under most OEM definitions (stop-and-go traffic, short trips under 10 km per cycle, ambient temperatures above 32°C). The recommended adjustment is to multiply the manual figure by 0.7–0.8 — so 15,000 km becomes 10,500–12,000 km in Thailand. Alternatively, collect a Used Oil Analysis sample (TBN, viscosity, soot) every 5,000 km first to verify the trend before extending intervals.

How long ago did API SP replace SN, and can I still use an SN-spec oil? API SP was officially licensed on 1 May 2020, replacing SN/SN Plus. SN-spec oil can still be used in vehicles whose manufacturer specifies SN and does not require LSPI protection. However, for 2018+ vehicles fitted with Turbo GDI engines (Honda 1.5T, Toyota 1.2T, Ford EcoBoost), SP is mandatory — SN does not include the Low-Speed Pre-Ignition (LSPI) or Timing Chain Wear tests that were added in SP.

Is Group III classified as Synthetic or Mineral? Technically, under the API 1509 definition, Group III is derived from crude oil via a severe hydrocracking process and is therefore mineral-derived chemically. However, in marketing terms, a 1999 New York Better Business Bureau ruling in the Mobil vs Castrol case permitted Group III to be labelled as "Synthetic". Today approximately 85% of "Full Synthetic" oils on the market use a Group III base blended with some Group IV (PAO). If you specifically need 100% PAO, look for the words "PAO-based" on the PDS, or product names such as Mobil 1 ESP or Amsoil Signature Series.

Can I use automotive Full Synthetic in a large motorcycle? No — not if the motorcycle has a wet clutch (which covers most motorcycles except scooters with CVT transmissions). Automotive API SP / ILSAC GF-6 oils contain friction modifiers (Resource Conserving) to reduce friction and improve fuel economy. In a motorcycle where the engine, gearbox, and clutch share a common oil sump, these friction modifiers cause clutch slip. Always use an oil certified to JASO T903 MA or MA2. Look for the JASO MA2 mark on the label. Scooters with CVT transmissions (Honda Click, Yamaha Aerox) are the exception — they use JASO MB, which allows friction modifiers.

Is premium oil at 800–1,200 THB/litre actually worth more than oil at 250–400 THB/litre? The answer lies in Total Cost per km, not price per litre. Example: Mineral 15W-40 at ~280 THB/litre × 4 litres / 5,000 km = 0.224 THB/km. Full Synthetic 5W-30 at ~950 THB/litre × 4 litres / 12,000 km = 0.317 THB/km. Synthetic appears more expensive — but when you factor in approximately 2% fuel economy improvement, reduced wear (extending engine life 20–30%), and fewer workshop visits, Synthetic delivers a better return for fleets covering more than 30,000 km per year.

Order & Inquiries

Sahawatthanakit (1988) Co., Ltd. — Engine oil supplier across Thailand, VAT invoices available.

• Tel: +66-2-096-2118
• Email: info@sahawatthanakit1988.com
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