is it dangerous to switch to synthetic oil?

I have a 2000 Saturn SL2, with about 72,000 miles on it. When I purchased the car, I was told to use 5w-30 oil and dont bother changing weights based on the season.

Well, it’s been really cold here in Boston the past few days (around 10 F), and when I start my car up, it’s not happy. It takes a few more turns to start, and is a little sluggish while the engine warms up.

I was talking with a coworker at work today about using synthetic oil, and he alluded to the idea that you can’t just go back and forth - it’s somewhat of a one-time deal. I’m not really concerned about that because if I switch, I dont mind continuing using synthetic oil. But one thing that was brought up in conversation was that it’s not necessarily good to switch after a car has been driven for a while on a particular type of oil, and has ‘gotten used’ to it.

So I’m wondering - at 72,000 miles, would it cause more harm then good to switch to synthetic oil on my next oil change? Is there anything in particular I need to do to ensure that I dont damage something? Or should I stick with the old petro 5w-30 oil because I’ve been using it for almost 4 years now.

I’ve occasionally heard people say that it’s not a good idea to switch back and forth, but have never been able to get any of them to explain why. Most seem simply to be passing on something they heard.

I called a Mobil 1 “consumer information” number about 6 years ago to ask this question, and was told that Mobil 1 is fully compatible with non-synthetic oils. Which sounds nice, but is perhaps a bit short of a fully authoritative view.

I’ve used Mobil 1 for New England winters for many years (superior for cold starting on days like today: -24 F). I used to switch to non-syn oil in the summer, and never had any problem - engines have all gone past 200k miles with flying colors.

I’ve taken to using Mobil 1 year 'round, so I no longer can report on the effects of switching.

FWIW, googling the topic produces the typical mishmash of info, with the general view predominating that compatability is not a problem. Here’s an extract:

*Synthetic Shell Oils CAN be Safely Mixed With Non-synthetic Products

Shell’s Staff Research Engineer Stephen Miller says he is at a loss to explain the growth of a myth which suggests that synthetic and regular motor oils can’t be safely mixed.*

There are synthetic lubricants which can’t be mixed with oil. These are usually polymers, and carelessly mixing them with mineral oil will cause the whole mixture to look like cottage cheese. Most (all, I think) motor oils contain synthetic additives, and multi-weight oils contain a large fraction of them. It’s apparent that Mobil 1 and similar consumer-oriented synthetics are freely compatible with mineral oils, probably because car engineers and petroleum chemists aren’t complete idiots, and they know that many consumers are. For more specialized applications (and more knowledgable clients), synthetic lubricants are available that aren’t compatible with mineral oil (I don’t know if any of of these are used in any kind of car, though).

You can go back and forth between synth and dyno without worries. It’s a myth when people say you can’t.

I’ve studied the oil debate long and hard, and for a guy like me who changes his oil monthly, it’s hard for me to justify the expense of synth oil.

I would always use the weight the manufacturer recommends - they know their products best.

Everything you ever wanted to know about oil:

I’m sorry, I posted the wrong link!

Here is the correct one:



My understanding of this is theoretical, but I’ve read that there can be issues with oil leaks when switching to synthetic oil in high mileage engines. The story goes that dino oil forms deposits over and through small leaks in gaskets which helps seal them. Synthetic oil, due to its higher detergency and fluidity, can break up those deposits and expose the voids, causing leaks.

Conversely, synthetics also contain additives that will condition seals, so I guess it is more a question of whether the seals will respond to the chemicals that should make them swell.