Should you use Conventional or Synthetic oil in your vehicle?

September 30th, 2012

I recently took my car to an oil change shop that really wanted to upsell me on expensive oil for my car.  I told them to put in whatever the dealer would use and they put in semi-synthetic.  This was a somewhat unfortunate illustration that working with a professional is no substitute for knowing what you're doing.  Though the semi-synthetic won't harm my car, the extra $20 won't offer any benefits, based on the car I drive, what I use it for, and the time of year.  Should you use conventional or synthetic oil in your vehicle?  Keep reading.

Sometimes, an auto manufacturer will prescribe a specific type of oil.  In that case, the decision is easy - use what the manufacturer prescribes.  Other times, the oil you use is somewhat up to you.

Synthetic oil can sometimes translate into longer oil change intervals.  Check with the manufacturer of the oil and the manufacturer of your vehicle.  For example, some (but not all) models of Toyotas allow you to double the oil change interval, if synthetic oil is used.  Be sure to also check the limitations of your vehicle's warranty or extended warranty, if it is under warranty.  Sometimes, even though the vehicle can tolerate a long oil change interval, the warranty prescribes a normal oil change interval, regardless of the oil used.  Remember that even if you needn't change the oil, you should otherwise service your vehicle as per the manufacturer's directions.  You should also check the oil level and add more oil if necessary.  (Source - Toyota)

You can switch back and forth from conventional to synthetic at any time, as long as the vehicle is not very old and has been properly maintained.  Even if the vehicle has high kilometers on it or hasn't had its oil changed regularly, you can use a short oil change interval for the first few changes.  (Source - Mobil)

Synthetic oil can offer a small advantage in fuel efficiency.  Based on the fuel efficiency of my car, this will translate into saving about $50 per 100,000 km.  Not huge, but not horrible either.  So why not use less fuel?  (Source - Toyota)

Synthetic oil will perform better when you attempt to start your vehicle on a very cold day, or if you use your vehicle at high temperatures and/or under high load, for example, if you tow things with your vehicle.  (Source - Canadian Tire).  The pour point of synthetic oil is typically better than conventional oil.  (Source 1 / Source 2 - Mobil)

Wikipedia cites a Consumer Reports study done on 75 taxis, published in 1996.  The results suggested that there was no advantage of using synthetic oil over conventional oil.  The author stated that one should consider using synthetic oil if they plan to operate their vehicle in "high ambient temperatures and [under] high engine load, or very cold temperatures".  (Source - Wikipedia/Consumer Reports)

It may be worth it to note that synthetic oil is not as expensive as the oil change shop wanted me to believe.  I priced the oils they use at a national automotive supply chain.  Mobil Super 1000 Conventional Motor Oil cost $5.89/L.  Mobil 1 Synthetic Motor Oil cost $8.39/L.  Based on the amount my car uses, synthetic would cost an additional $8.75 if I changed the oil myself, but an additional $35 if I had it professionally done.  Caveat emptor.

As for me, I plan to use conventional oil in spring, summer, and autumn, and synthetic in the winter, until my warranty expires.  Then I'll use synthetic oil for all seasons and take advantage of its longer oil change interval.  And it sounds like it would be a financially sound idea for me to learn how to change my oil.
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