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Full synthetic oil

Why can’t you switch back to regular oil once synthetic oil is used?


yes, why? it’s not a one way street.

There may have been some validity to that claim maybe 30 years ago but not since.

Synthetic and regular mineral oils can be mixed or one can be used after the other. Just use the proper weight (viscosity) and make sure the oil meets the spec for you vehicle.

I understand the reasons for not using synthetic oil if you have someone else change it for you. There is often a HUGE markup on synthetic vs. conventional. If you are like me and pickup several changes of synthetic during sales and rebates, you can get it for the same price or less than conventional. I paid $12 for my last two 5 quart jugs of Mobil 1 EP. Why not get it if you can get it for less money than conventional, especially since it is a better product?

As for mowers and such, I think that synthetics make great sense on air-cooled engines that tend to run hotter and have more uneveness to their hot spots than water cooled units. I like the Rotella diesel rated oils for engines that have flat tappet lifters and don’t have emissions equipment to foul from additives like a street legal car.

I seem to recall that using synthetic oils in small aircraft engines burning leaded fuel is a big no-no. I forget why but seem to remember that the lead additives in the gas react poorly with the oil, causing deposits or something???

I wonder what Tom McCahill would say about synthetic oil if he was living today. In his book “What You Should Know About Cars” published over 50 years ago, he didn’t like detergent oil. He said, “I prefer detergent in my bathtub, not in my crankcase”. He didn’t like multi-viscosity oil either. He said, “10W-30 is a lousy number 10 and a lousy number 30”. He went on to say that detergent oil might be useful for keeping hydraulic lifters quiet, but he preferred solid tappets and a properly adjusted solid lifter valve was as quiet as any hydraulic lifter operated valve. He claimed that the real reason for multiviscosity oil was so service station operators wouldn’t have to have an inventory of single viscosity oils. I wonder what old Tom would find wrong with synthetic oil.

Dino oil and synthetic oil are chemically the same. The differences are that synthetic oil has fewer impurities and allegedly more consistent molecule size. The former reduces susceptibility to the effects of high heat, and the latter reduces shear damage and reduces susceptibility to the effects of extremely low temperatures in the extreme sub zero range.

Because of these differences, synthetics are required by manufacturers for engines that subject the oil to extreme stresses, such as those that lubricating the bearings in a turbocharger subject the oil to. For engines that don’t subject the oil to extreme conditions, in driving environments that don’t subject the oil to extreme conditions, synthetic has no technical benefit. BUT, if it helps the owner sleep better, that is benefit enough all by itself to justify the added cost. Placebos DO have value.

Switching back and forth, or using blended oil to “hedge one’s bet” for engines that don’t require synthetic, are totally harmless. The myth that once you switch you can’t go back is just that… a myth. When something new that people don’t fully understand comes to the market, myths are a perfectly normal reaction.

Any oil change in time is a good oil change. Not using synthetics, regular stuff, 170k and still ok.

I think the original synthetics and engine seals of the time had issues with going back and forth but those days are over.

Interesting we just got back from an Acura customer workshop. Explaining the service codes, the one means change the oil and the other means change the oil AND filter. Huh? I haven’t done this for 20 years like we used to change filters every other oil change. All I can say is not in my car!

Down fall is you can't switch back once using full synthetic oil.

Sure you can. That’s just urban legend. You can go back and forth and mix and match any way you want with no adverse effects what-so-ever.

One other oil property is film strength. Synthetics have conventionals beat, especially in the thinner grades like 0W20. This isn’t as much of a concern in the Jeep 4.0L that started this thread off but manufacturers are trying to squeeze all the MPG possible out of their new models and one way to do this is with thinner oil. I feel (as do others) that eventually synthetics will become the norm and conventional will become rarer.

As for the blends, I have never really been a huge fan either but have some friends who run them, including a car that is raced, and never have any complaints.

One thing to note if you switch a car from conventional to synthetic is that the first oil change with synthetic will get dirty quite quickly if there is any accumulated sludge in the engine. You might not push the first oil change with synthetic too far. Look at the stick and see if it turns black quickly.

I would prefer buying a car from somebody who has run a car exclusively on Mobil-1 EP (and can document this) rather than any old conventional oil, considering both are changed at manufacturer’s recommended intervals.

It says to me that the owner cared about the car and its longevity and I really believe could help command a little higher resale price or make it easier to sell.


“One thing to note if you switch a car from conventional to synthetic is that the first oil change with synthetic will get dirty quite quickly if there is any accumulated sludge in the engine. You might not push the first oil change with synthetic too far. Look at the stick and see if it turns black quickly.”

I noticed that with my lawn mower. I actually changed the syn out at about 15 hours instead of 25 because it looked a little dark. Makes sense that it was cleaning the junk out a little.

Oh no…here we go again…

And clearly somebody that uses conventional oil, checks the level weekly, and changes it every 6 months/3K, whatever comes first . . . does NOT care about the car and longevity

Never mind they keep the car in immaculate condition mechanically and visually

Never mind they may keep impeccable records

Never mind they only do freeway driving

Clearly using conventional oil says the owner is just a bad person. Period


Now I’m feeling guilty that I didn’t use synthetic oil on my 72 Super Beetle that I drove for 35 years.

" Down fall is you can’t switch back once using full synthetic oil. "

One reason why some myths never die, someone just has to keep bringing them back from the dead.

Why you saying that @db4690 ? I dont get it. Nothing wrong with Conventional oil…some vehicles do require full synth…but not all. Again…nada wrong with Good Ole Dino oil. I must have missed another comment you are referring to LOL… Wouldn’t be the first time for me…

@Honda Blackbird

You thought I was SERIOUS. . . ?!

come on, man . . . !

perhaps I should have used this emoticon instead :smirk: