01
Definition
When crude oil is distilled the lighter fractions become petrol, kerosene and diesel. The remainder, refined further, becomes base oil — the main raw material of lubricants.
Base oil makes up most of the finished product — over 90% in industrial oils — with additives making up the rest. That is why base oil quality dominates the quality of the finished oil.
Base oil refined from crude is called mineral oil; base oil built by chemical reaction is called synthetic oil.
02
API base oil groups
The American Petroleum Institute sorts base oils into five groups by saturates content, sulphur content and viscosity index. Higher groups are purer and more stable.
| Group | Saturates | Sulphur | Viscosity index | How it is made |
|---|---|---|---|---|
| Group I | น้อยกว่า 90% | มากกว่า 0.03% | 80–120 | กลั่นแยกด้วยตัวทำละลาย (solvent refining) |
| Group II | ตั้งแต่ 90% ขึ้นไป | ไม่เกิน 0.03% | 80–120 | ไฮโดรทรีตติง (hydro-processing) |
| Group III | ตั้งแต่ 90% ขึ้นไป | ไม่เกิน 0.03% | ตั้งแต่ 120 ขึ้นไป | ไฮโดรแคร็กกิงขั้นสูง — YUBASE อยู่กลุ่มนี้ |
| Group IV | PAO (Poly Alpha Olefin) | — | — | สังเคราะห์ทางเคมี |
| Group V | เอสเทอร์ ไกลคอล และอื่น ๆ | — | — | สังเคราะห์ทางเคมี |
SK’s YUBASE is a Group III stock made by severe hydrocracking, giving a viscosity index above 120 (the VHVI band). Most oils marketed as “fully synthetic” today are built on Group III.
03
Classification by viscosity index
Viscosity index (VI) describes how much viscosity moves with temperature. The higher the VI, the more stable the oil — not too thick when cold, not too thin when hot.
| Class | VI range | Meaning |
|---|---|---|
| LVI | ต่ำกว่า 40 | ดัชนีความหนืดต่ำ |
| MVI | 40–80 | ปานกลาง |
| HVI | 80–120 | สูง — เบสออยล์น้ำมันแร่ทั่วไป |
| VHVI | 120–140 | สูงมาก — YUBASE อยู่ระดับนี้ |
| UHVI | มากกว่า 140 | สูงพิเศษ |
04
How base oil is made
- Hydro-cracking (HCK)Breaks down large molecules and strips out sulphur, nitrogen, oxygen and aromatics — raising viscosity index, oxidation stability and colour.
- Hydro-finishing (HDF)A final polish that removes what is left, improving clarity and stability further.
- Catalytic de-waxing (CDW)Removes wax so the pour point drops and the oil still flows when cold.
05
How base oils are named
- Neutral (N) — vacuum-distilled stock neutralised after refining; the number is an approximate viscosity, such as 150N or 500N.
- Bright Stock (BS) — heavy stock from the residue, de-waxed and hydro-treated, blended in to add viscosity, such as 150BS.
- Trade names such as YUBASE 4 and YUBASE 6 — the number is viscosity at 100°C (4 cSt and 6 cSt).
| Base oil | Viscosity at 40°C (cSt) | Viscosity at 100°C (cSt) |
|---|---|---|
| 70N | 13.3 | 3.0 |
| 100N | 21.5 | 4.0 |
| 150N | 31.6 | 4.9 |
| 250N | 56.1 | 6.5 |
| 500N | 107.0 | 11.0 |
| 600N | 130.4 | 12.1 |
| 135BS | 413.2 | 28.6 |
| 150BS | 568.0 | 33.0 |
| 175BS | 654.7 | 36.0 |
Adapted with permission from the technical material of SK Enmove (skzic.com) · Standard values referenced from SAE J300, SAE J306 and ISO 3448