Synthetic vs Conventional Oil: What's the Real Difference?

Conventional oil is refined crude with additives. Full synthetic is built from uniform, engineered base molecules. That uniformity gives synthetic better flow when cold, far better resistance to breaking down when hot, and much stronger protection against sludge and varnish.

• Turbocharged engines — turbo bearings cook oil, and conventional oil coking is a leading cause of turbo failure.
• Direct-injection engines, which suffer more fuel dilution of the oil.
• Very cold climates: synthetic flows at start-up when conventional is still thick.
• Extended drain intervals — a synthetic's additive package lasts longer.
• Towing, track days, and sustained high-load driving.

An older, naturally aspirated engine in a mild climate driven on a 5,000-mile interval does not suffer on conventional oil. If the manufacturer specified conventional and you change it on schedule, you are protected.

Note that most cars built in the last decade specify synthetic outright — usually because the specified viscosity (0W-20 and thinner) can only be produced as a synthetic. In that case it is not a choice.

Conventional: typically 3,000–5,000 miles. Synthetic blend: 5,000–7,500. Full synthetic: 7,500–10,000, and up to 15,000 on some European long-life specifications with the correct oil.

Follow your oil-life monitor if the car has one — it counts engine conditions, not just miles. Short-trip driving where the engine never fully warms up shortens the interval regardless of what oil is in the sump.

You can move between conventional and synthetic in either direction at any mileage. The old belief that synthetic causes leaks in high-mileage engines came from early formulations; modern synthetics include seal conditioners. High-mileage synthetics add more of them.