Walk into any auto-parts store and you're staring at a wall of options: conventional, full synthetic, synthetic blend, high-mileage — with prices varying by a factor of three. This page cuts through it: what each oil type actually is, where the differences matter, and where they don't. It's part of our DIY oil change hub; if you're still working out viscosity, start with what oil does my car need.
It All Starts With Base Oil Groups
Every engine oil is a base oil plus an additive package. The API classifies base oils into five groups: Group I–II (solvent-refined and hydro-processed mineral oil — most conventional oil), Group III (severely hydro-cracked mineral oil, highly refined — permitted to be sold as "synthetic" in most markets, which is why some synthetics are cheaper than you'd expect), Group IV (PAO, purpose-built synthetic molecules — true full-synthetic territory), and Group V (esters and others, usually blended in). So "synthetic" on a bottle doesn't guarantee Group IV PAO — for most cars a distinction without a huge practical difference, but it can matter for a high-revving performance engine.
The Four Oil Types, Plainly
Conventional — Group I/II base, fewer processing steps. Breaks down faster under heat and leaves more deposits, but fine for older engines on shorter drain intervals and mild driving. Synthetic blend — a mix of conventional and synthetic base; better thermal stability than conventional at a lower price than full synthetic. The catch: the ratio isn't disclosed, so if the price gap to full synthetic is small, go full synthetic. Full synthetic — Group III or IV with a sophisticated additive package; uniform molecules mean better cold flow, higher breakdown resistance and longer life. The right call for modern turbos (which run very hot), performance/track use, tight tolerances and extended intervals. High-mileage — built (on conventional or synthetic base) for engines over ~120,000 km, with seal conditioners that swell aged seals back toward shape, plus extra detergents and anti-wear additives.
The Switching Myth
Can you switch to synthetic? Yes. The idea that switching causes leaks is a persistent myth with essentially no basis in modern oil chemistry — it dates from the aggressive ester-based synthetics of the 1970s and the seal compounds of that era, neither of which exist now. What can happen on an older engine: full synthetic's superior cleaning can loosen deposits that were incidentally sealing a weeping seal, making a pre-existing leak more visible. The synthetic didn't cause it. If your engine has high mileage and you want to switch, a high-mileage full synthetic is a clean transition.
Drain Intervals by Oil Type
| Oil Type | Normal | Severe Service |
|---|---|---|
| Conventional | 5,000–8,000 km | 4,000–5,000 km |
| Synthetic blend | 7,500–10,000 km | 5,000–7,500 km |
| Full synthetic | 10,000–15,000 km | 7,500–10,000 km |
| High-mileage synthetic | 8,000–12,000 km | shorter if consuming oil |
Always follow your manufacturer's specified interval as the maximum, regardless of bottle claims — full breakdown in oil change intervals.
Cost-Per-Kilometre: The Number That Matters
| Oil Type | Cost (4 L) | Interval | Per 1,000 km |
|---|---|---|---|
| Conventional | $18–$28 | 6,000 km | $3.80–$4.70 |
| Synthetic blend | $28–$40 | 9,000 km | $3.10–$4.40 |
| Full synthetic | $45–$70 | 12,000 km | $3.75–$5.80 |
| High-mileage synthetic | $50–$75 | 10,000 km | $5.00–$7.50 |
The per-kilometre gap between conventional and full synthetic is much smaller than the per-bottle price suggests. Factor in that synthetic runs cleaner and keeps the engine in better condition, and it's hard to argue against it in most modern cars.
The Short Version
Conventional if your car is older (pre-2000), non-turbo, the manual specifies it, and you change it on the shorter schedule. Synthetic blend if you want better than conventional at a lower price than full synthetic — light trucks, moderate towing. Full synthetic if your car is from the last 10–15 years, turbocharged, driven hard, in temperature extremes, or you want extended intervals. High-mileage synthetic if your car has 120,000 km+ and shows consumption or minor seep. Pair your choice with the right grade from what oil does my car need, and the full service is in the oil change hub.
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