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I've been storing my 2009 Infiniti G37 since 2010, when my father passed away and I inherited his 1999 Chevy Tahoe. The car has 21,000 km on it and the truck has 223,000 km. I plan to switch to synthetic oil for both vehicles. Should I flush the engines before I replace the oil and filter?

– Joseph

It's a bad idea to flush an engine just because you're switching oils – synthetic and conventional can mix just fine, says a BCIT expert.

"A flush here is not required in either vehicle," says Eugene Eng, professor in motive power at BCIT in Burnaby. "The two oils are miscible, meaning they can mix together in any ratio."

Engine flushes aren't really for flushing oil. They act like drain cleaners, washing carbon deposits out of your engine. In an engine flush, the technician removes a bit of the oil, adds the flush additive and runs the engine. The chemical loosens all the, er, crap. It gets washed into the engine's oil and caught in the oil filter. Then the oil is changed normally.

"It was once commonplace to add automatic transmission fluid to oil and run it before oil changes, as the detergent in the fluid would serve the same function," Eng said. "It's absolutely not a recommended procedure, but the idea was been around for decades."

Synthetic and conventional can mix, so you don't need to get all of the previous oil out.

"Even flushing does not remove 100 per cent of engine oil," Eng says.

Oil already has detergents, but they're not there to clean your engine – they're there to keep carbon particles circulating in the oil, so they don't form gunk in your engine. Bob is the Oil Guy does a good job at explaining this.

If you don't get regular oil changes, those particles can build up – and an engine flush might not be a bad idea.

"If the high-mileage Tahoe had not been receiving regular oil changes, it may benefit from the flush," Eng says. "The detergent and emulsifiers in the flush additive can help break up any sludge deposits circulating and suspended in the oil."

Switching to synthetic isn't a problem, but is it a good idea? Eng thinks switching to synthetic for the Tahoe, which was designed to run on conventional engine oil, isn't worth the extra cost. "If it's been running on conventional for 225,000 kilometres already, the majority of the wear the engine will suffer has already taken place," he says. "In my opinion, switching won't be a good return on investment."

How do you decide whether it's worth it? Figure out how long you want to keep the Tahoe and calculate the difference between conventional and synthetic oil changes.

A synthetic like Royal Purple can cost $15 per litre, compared to conventional oil at $3 a litre. If each oil change takes five litres, synthetic costs $75 an oil change and conventional costs $15. That's an extra $60 per oil change. If you change your oil every three months for ten years, that's 40 oil changes, Eng says.

"The difference in ten years is $2400," he says. "You could do a lot of engine work for $2400 on a 5.7L if you haven't sold the car, wrecked it or had overheating."

Conventional oils designed for vehicles with high mileage might be a better bet for your 15-year-old Tahoe, Eng says. And if you see blue smoke on start up, it's burning oil and you should switch to a heavier oil, he says.

"As rings and seals wear, thinner oils have an easier time sneaking by and into the combustion chamber," he says.

Eng says Nissan recommends ester-based synthetic oil for the Infiniti.

"Many owners of the VQ engines complain about knocking when using other oils," he says. "Although more expensive than even most synthetic oils, ester has proven to be the best and quietest for those vehicles."

While storage can cause problems with carbon build up, Eng doesn't recommend a flush for the Infiniti if it's had oil changes done on schedule.

Eng figures synthetic will be worth the investment on a relatively new car with such low mileage and an engine that's more expensive to fix than the Tahoe's.

"The G37 engine also has cam phasers and variable valve timing, a much more elaborate timing chain setup, over-head cams and low friction engine materials – and they benefit more from the proper oil," he says. "It's why the engine runs more quietly with ester."

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Correction: An earlier version of this story misquoted Eugene Eng. He compared the price of conventional oil at $3 per litre (not $5 as in the earlier draft) to Royal Purple synthetic at $15. The story has been corrected.