Synthetic vs. Standard oil change schedule


I overheard Tom and Ray’s recent show and something about how many miles before an oil change for synthetic oil vs. conventional. I recall them saying that some 30,000 miles minimum may elapse before an oil change is necessary with synthetic oils. Did I hear this right? I went out and bought some synthetic oil for two of my five cars to give it a try, and asked the parts salesperson at Pep Boys what he thought of how many miles before a change. He said only 6000 before a change is needed ( I bought Castrol fully synthetic… they also carry a cheaper synthetic blend…) He told me to also buy the premium oil filter for this purpose, which I did (Fram “Tough Guard” series). Now I have some questions:

how often to change this synthetic?

how often to change the filter?

what about the dirt and engine debris, which I assume still will accumulate in the oil… where does it go after so many miles and isn’t it good to get this stuff outta there? (conventional oil gets dirty with this stuff the filter cannot remove, so why would it be different with syn?

what grade for my climate/auto type/age (Los Angeles, 38000 mi 2004 Toyota v6 highlander and 130000 mi 1999 Ford F250 Super Duty)… I bought 10w40

do I need to gauge the oil changes based on physical inspection of the oil? (as my salesman said, as he brought me to a display device of gears in a see thru housing that showed the difference in tackiness of new and worn oils).

I would like to go 30000 or more on a change. Anyone behind me?

Oh, and what happened to the synthetic oil type I used long ago… it was black and like graphite straight out of the bottle… is this still sold? My new syn is unexpectedly like regular oil.


Change your oil and filter in accordance with the schedule recommended in your owner’s manual despit what anyone else says.

Engine oil deteriorates from three basic causes. (1) the oil becomes contaminated by particulates from the parts it’s lubricating. Not all of these are filtered out. (2) oil becomes diluted by and contaminated by “blowby”. Blowby is gasses and chemicals being blown past the pistons rings during combustion. Some of this occurs in all engines. (3) oil suffers from “shear” and from heat damage. Visit for these descriptions.

All oil suffers from 1&2. Synthetic suffers less from 3.
If you want your engine to last, change your oil regularly. If you’d prefer that your oil last longer at the risk of your engine not lasting as long, go to extended schedules. The choice is yours.

In summary, are you switching because you want your engine to last longer or because you want to do fewer oil changes?


If your car is still under any kind of warranty, stay with the intervals specified in the owner’s manual, and do not use synthetic unless your owner’s manual Oks it.

Beyond that, you will get many different opinions. Which is correct? I don’t know, but I don’t want to take a chance on a $4000 engine for lack of a $50 oil change.

I had someone tell me to change the oil filter without changing the oil (with synthetics). Someone more knowledgeable than a Pep Boys salesman, but I still discounted it.


On the other hand, the intervals specified in the owner’s manual are partially dictated by politics, by marketing pressure, and by government regulations. How different they would be if they were specified just by the engineering staff, I don’t know, and lacking a pipeline into the manufacturer’s engineering department, the owner’s manual is all we have to go by.


There really is no neutral authoritative source of information on your questions. Oil manufacturers want to sell more oil. Filter manufacturers want to sell more filters. Car manufacturers hope to avoid giving away new engines on warranty. Advice givers (us) don’t want to urge our best customers to take any unnecessary risks with their engines. So even if it were indeed true that you could safely go 30,000 miles between synthetic oil changes, who in his right mind would urge you to do so?

In short, we just don’t know. Considering what’s at sake it is surely best to play it safe.


Well said.

There are no objective controlled tests that I’m aware of corrolating the differences in engine wear and/or longevity between sythetic oil and dino. And I’m unaware of any body of studied empirical data.

When in doubt, err toward the extra oil changes.


If you want to pursue this I suggest that you get your oil analyzed by a professional laboratory. That is the only way to tell if it is used up.


I would never try 30K miles, but Amsoil states their expensive synthetic will go 25K miles. What I do is use synthetic for 7500 miles and regular or semi-syn for 5000 miles. Note that Ford recommends your oil should be changed every 5K miles, regardless of whether it is conventional, semisyn or syn. That has been true for 95 - 02 one ton models I own. Based on that I use Motorcraft oil and Motorcraft filter, because Walmart provides both at a good value. For our passenger cars, I use synthetic to the 7500 miles easily.

Today’s oils are extremely good and last longer. I am too cheap to get a regular oil analysis, but if I ran to 25 or 30K, it would be on my list to get done, to make sure the oil can actually take 25K miles, or more.

Your choices may vary. Good luck…


I like your answer, and all others thus far. It must be mentioned that I felt nervous about the idea of going 30000 without a change, but I liked the idea of not having this task be so frequent (after all, I have five cars now, and have been doing oil changes for 35 years… it gets a bit old). I pride myself on helping engines last long, and I feel there is a vehicle’s soul that sighs relief every oil change. I did not mention that all vehicles are out of warranty, but this is one primary factor in going longer between changes. I think I will do this one synthetic change this time and then go back to regular oil at regular intervals just because I am old hat. And what is synthetic oil anyway? I would assume it is something out of a lab instead of out of the ground. I wonder if when oil is really expensive will it catch up with the price of synthetic? and is this black graphite stuff still out there?
Thanks much for your comments


I agree with the oil analysis recommendation, you don’t have to do it every time, just once or twice to establish the correct oil change interval.


You are free to experiment by changing just the filter at 5000 mile intervals and topping off the oil but I don’t have any real statistics to offer except that Castrol seems to be really proud of that oil and had a great infomercial without that shouting buffoon that you see everywhere now.They didn’t bless it with a bishop but I don’t think you will be rooked no matter what you decide to do. I always wish that other people would do the experimenting while I change it every 3000 miles like my owner’s manual recommends for the GMC 4.8. In the winter, I have a service station do my oil change and lube and they use Kendall oil, non-synthetic. I let the Toyota dealer change the oil on the Yaris and I don’t bother asking what oil they use. I have only lost the engines that were ran without oil.


Back around 1968 when I first began working in a gas station, a customer came in for gas and asked me to check the oil. It wasn’t low but it was VERY black. I told the driver he should get it changed. He replied saying he only changes the filter every 25000 miles, never changes the oil, and keeps his cars for 100,000 miles. I recall his car then had 87000 miles on it.

There are countless car owners out there who semi-intentionally neglect to maintain their cars, including oil changes. Then they sell their cars before any fallout from neglected maintenance shows up (say 5-8 years). They do fine.

While that’s not the approach I take, I can’t argue with their being entitled to treat their car the way they want to.


Goto read up there and post. People live and breath all this stuff. Basically you will find if you want extended oil changes you need to analyze your engine oil ($20) by a lab to see if its viable or not. Hardly worth it when you only have 4-5qts in the sump, however a different story when your engine(eg ship, locomotive etc) requires gallons or barrels of oil.


Back around 1968 when I first began working in a gas station, a customer came in for gas and asked me to check the oil. It wasn’t low but it was VERY black. I told the driver he should get it changed. He replied saying he only changes the filter every 25000 miles, never changes the oil, and keeps his cars for 100,000 miles. I recall his car then had 87000 miles on it.

That was actually Greyhounds policy for years (not sure if they do it now). They would NEVER change the oil on their busses…Only add when needed and change the filter every 3-5k miles. I sure don’t recommend that.

As for the 30k mile change interval. You heard it wrong. They never said that. I don’t know any oil that will last that long. Convential oil I’d change every 3-5k miles with filter. Synthetic I’d change every 5-7k miles with filter. Even with oil prices very high…it’s far cheaper to change your oil every 5k miles then it is to buy a new engine every couple of years.