Full synthetic and Regular oil?

So I have a friend who doesn’t take good care of his things. Was driving in his truck today and asked him when’s the last time he had his oil changed as the sticker showed he was several thousand miles over. He said that this time around he got full synthetic and they pit the traditional 3k mile sticker on instead. He then went on to say he switches between full synthetic and traditional oil all the time just based basically on which way the wind blows. I then proceeded to call him an idiot and explained how that’s bad for your engine. This opinion based on what mechanics have explained to me in the past. So I internet searched it and the internet said that the idea that changing your type of oil whether it be one time or often, being harmful for your vehicle is a complete myth.

So which is it? Switching back and forth between synthetic and traditional is a problem or no problem at all?

I am no engine oil wizard but I don’t see how it could be harmful.
Oils primary job is to lubricate your engine parts.
Both synthetic and conventional oil do this job. One breaks down quicker then the other so that is why the lifespans are different.

I myself haven’t ran conventional oil in years. But there has been times I went back and forth between full synthetic and partial synthetic depending on what the store sells.

Answer. Synthetic oils typically provide better protection than conventional oils, but switching back and forth between full synthetic and conventional oil will not damage the engine. That came right from MOBIL. Personally I’m with you , I run full synthetic and that motor will only see synthetic .

I run synthetic all winter and conventional in summer. Little different than some, but this is in my diesel trucks. Ran two Duramax over 3/4 of a million miles and they both were solid when I sold them. Neighbor kid pulled the LB7 and dropped in another truck and it’s still running. My other two trucks are both under 500k miles but going strong. However I do add Lucas oil stabilizer for the hottest few months of the year.

I did the same with my gasser Toyota Corolla. Sold it with 360+ K miles. Engine purred perfectly when I sold it. That little 4 banger had nearly as much zip to it as when it was new

Thanks guys. Sounds like I am the idiot

The theory is that if you run conventional in an engine for a long period of time then switch to synthetic, the engine may start to leak. Conventional traditionally has higher viscosity at cooler temps and is less likely to leak at cooler temps. Synthetic on the other hand has much less viscosity at cooler temps. One of the biggest advantages is that engines are much easier to start in cold weather with fully synthetic oil.

What happens is when the engine cools, parts and gaskets contract and can reveal slight gaps in the gaskets allowing synthetic oil to leak through when conventional may not leak through because it is much thicker (viscous).

Bad for the engine? No. It just might start to leak with synthetic. This was more of a problem 20 years ago when conventional oils weren’t Calvert good compared to synthetic. Today most conventional oils are far better than what they used to be.

The only thing that matters is the api rating. If both conventional and synthetic have the same api, they are compatable.

For the record when you guys say conventional oil it is most likely a synthetic blend you are speaking of. You can still buy conventional oil but it is now less common and most times more expensive to than the blend. At my shop and all the others I know of stock blend and full syn. As far as switching No Crappie you not an idiot for saying it, on gas engines I would still tell people to stick with one or the other, while it may not damage things it would still be best to stay with one. Also agree with Matt on the leaking aspect with full syn after using conventional or blend.

Blaming synthetic oil on what would simply be a worn engine is wrong.

Deuces wrote:

Blaming synthetic oil on what would simply be a worn engine is wrong.

When I did cold weather vehicle testing during college the engineers said they didnt use synthetic oil in the test vehicles because they wanted to see if things would break. We all know that engineers are always right.

Just an observation from doing all my own oil changes in my trucks (diesel)

Pulling box trailers and equipment obviously requires me to run the engine a little harder. Running fully synthetic in the lighter weight for winter is the only time I ever see that I used any oil. Maybe 1/2qt if 10 or 12qt capacity and over about 7500-8000 miles. 1/2 is probably the most, more like 1/4-3/8 qt

Randy Wieland wrote:

Just an observation from doing all my own oil changes in my trucks (diesel)

Pulling box trailers and equipment obviously requires me to run the engine a little harder. Running fully synthetic in the lighter weight for winter is the only time I ever see that I used any oil. Maybe 1/2qt if 10 or 12qt capacity and over about 7500-8000 miles. 1/2 is probably the most, more like 1/4-3/8 qt

When we would swap an engine from conventional to synthetic we would always tell them to either check the oil a few times or stop by and have us check it. It would always burn a quart or two for the first oil change.

Does not matter at all on an engine designed to run either, you can “blend” your own in the same change if you want ! Some vehicles “require” synthetic at a specific ratio… but otherwise, myth.

If I recall right, back when synthetic oils came out there were some compatibility concerns that switching a conventional motor to synthetic and then back to conventional could cause excessive sludge buildup in the engine. I think they have disproved that or eliminated that concern.

Last time I asked a oil change place about their blend they could not tell me percentages.
What I have heard about viscocities is for dino oil the viscocity is an overall average of what is is the container, with synthetic all the molecules are exactly the same which can lead to more leaking because their are no larger particals getting stuck in tiny gaps. This from an art teacher, take it for what it’s worth!

tim hurley wrote:

with synthetic all the molecules are exactly the same

The art teacher is correct Tim. That is why synthetic is better it flows through the engine faster and smoother. But on a leaker of older engine it can cause leaks.

I’m no oil engineer, but live by the idea of keeping things simple when I can.

For me, I like consistency and fewer variables when possible. If I use the same product over and over and it works well…then it’s easier to identify when issues happen or if there are noticeable changes – especially on older vehicles/equipment.

Synthetics and blends are better when used consistently, especially from new. Most wear in an engine happens during a cold start… it takes time for the oil to coat every surface… synthetic is better at this than conventional. But like said, with worn or dried out gaskets, conventional tends to help “seal” better than synthetic. Also, just my opinion but… if you have a transmission with a ton of miles on it, seems fine and has never been serviced/flushed… “DON’T”. It tends to free up otherwise stuck debris in the nooks and crannies and causes issues.

I would not assume the blend is 50/50 unless they confirm that it is.