What happens if i put conventional oil instead of synthetic

File photo of BMW luxury car logo's in a spare part store at a BMW garage in Niderwangen near Bern, May 24, 2012.Pascal Lauener/Reuters

I drive a 2007 BMW 650i. My car recently sent an alert that the oil level was low. I added a high quality (non-synthetic) motor oil, then realized the car's oil is a synthetic oil. So now I have a bit of a mixture of synthetic and non-synthetic. Is this a problem? I drive a 2007 BMW 650i. According to the car's info, I rarely need an oil change. – Larry

Don't be too worried about adding a little conventional oil to a car's synthetic oil – as long as it's just a litre or so, oil's well that ends well.

"While mixing synthetic with regular petroleum-based oils is not recommended, when a top-up is required and only non-synthetic oil is available, it is acceptable to mix," wrote BMW Canada spokesman Robert Dexter in an email. "It should not result in any engine damage – with the proviso that the recommended viscosity rating must be adhered to."

You don't need to rush out for an oil change if you've mixed the two, Dexter said.

"However, if the vehicle hasn't had an oil change in some time, it would be a good recommendation to expedite the schedule for the next service."

Engine oil falls into one of three categories: petroleum-based mineral oil (conventional), synthetic oil and semi-synthetic, a mix of the two.

If adding a little conventional oil to your engine is okay, why not just switch entirely to save a little money? Some experts say you can.

"As long as the oil meets the service and viscosity requirements set out in your owner's manual, you can switch back and forth as much as you want," Edmunds.com engineering editor Jason Kavanagh, wrote in a story.

The AAA's Mike Calkins disagrees. Calkins says it's fine to switch between conventional and synthetic if your car is supposed to use conventional oil – but, if your car's manufacturer recommends synthetic oil, you should use synthetic oil.

"You can always step up a grade, but you should never go the other way around," said Calkins, manager, technical services. "Conventional oil is not as heat stable – engines are designed to use a certain type of oil."

High performance and turbocharged engines run hotter and often call for synthetic oil.

Calkins explained that conventional oil, made of long chain molecules, breaks down over time and with exposure to heat, causing it to become thinner and lose its ability to lubricate. The molecules in synthetic oil are engineered to resist breaking down, allowing it to stay stable at higher temperatures compared to conventional oil.

"That's the key difference," he says. "But synthetic oil usually contains a higher quality of additives to clean the engine, prevent viscosity breakdowns, and fight acids."

BMW Canada's Dexter says regular mixing of conventional and synthetic isn't a good idea because the additives in conventional oil "destabilize the additional properties of synthetic oil."

So what will happen if you ignore the recommendations in the owner's manual and switch to conventional oil?

"The engine won't blow up or anything," Calkins said. "But you'll see increased wear and build up of deposits."

And again, topping up with conventional oil in a pinch is okay.

"Don't panic if it's up to a quart," Calkins says. "It's not the end of the world."

Is synthetic oil worth the extra cost if your car calls for conventional? Getting an answer is slippery. Experts debate claims that synthetic oil delivers better fuel economy.

Consumer Reports said synthetic can mean less frequent oil changes, which can help balance out the extra costs. The AAA's Calkins said there's more to synthetic oil than just slick marketing.

"In the old days, as long as you used the best conventional oil, it was good for everything, except for diesels," he said. "Now engines are built to incredibly tight tolerances – if it's designed for synthetic, then it's necessary."

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