What happens when you mix oils
Modern motor oils that meet recognised specifications are formulated to be compatible with one another. Mixing two reputable oils — different brands, or synthetic with conventional — does not normally cause a harmful reaction, sludge, or sudden damage. In a pinch, mixing is far safer than driving with the oil level too low.
The real issue is dilution. Each oil is a carefully balanced blend of base oil and additives. When you combine two products, you also combine and dilute their additive packages and base stocks. The resulting mixture may no longer precisely match the grade printed on either bottle, and its overall performance becomes harder to predict.
Brands, grades, and synthetic versus conventional
Mixing brands of the same grade and specification is the least disruptive case, because both oils are aiming at similar targets. Mixing grades — say 5W-30 with 5W-40 — produces a blend that sits somewhere between the two and may not meet either grade exactly. Mixing synthetic with conventional simply gives you a partial blend; it does not ruin the synthetic, but it does dilute its advantages.
In every case the mixture is a compromise. It can get you home or buy time, but it is not a setup to run on indefinitely.
The manual-first rule
The best practice is straightforward: use a single oil that meets the viscosity grade and the specification (API, ILSAC, ACEA, or OEM) listed in your owner’s manual. That way the additive balance and properties are exactly what the manufacturer intended.
Treat mixing as an emergency measure, not a routine. If you have had to top up with whatever was available, that is fine for getting to a safe place — just return to your manual’s specified oil at the next oil change.