Oil & Filter Change Intervals

Engine Oil Change Intervals

There exists what seems to be an overwhelming amount of opinions on oil change intervals. Some say to follow the owner’s manual, that the engineers designed the engine and they know best. (This might be true if it really wasn’t the marketing departments overriding what the engineers actually say.) Others say to take what the manual says and to divide that in half. Then you have service technicians and engine rebuilders telling you to change your oil every 3-5,000 miles. With all that confusion, it takes what should be an easy matter and turns it into an extremely difficult one, leaving you to not know who to believe.

Before modern engine oil evolved into what it is today, conventional oil was advised to be changed every 3,000 miles to ensure optimal performance, protection and cleanliness. Eventually, synthetic oils were created, as were much more robust additive packages. However, the additive package alone is not everything; that is, the oil’s color also matters. Some will tell you darker oil doesn’t mean it’s bad, and while they may be correct, darker oil also directly indicates the amount of carbon suspended within the oil, and this quantity is limited to oil’s holding capacity. Therefore, in example, while the additive package could very well sustain at least 5,000 miles before needing to be replaced, excessive carbon contamination may occur before then, allowing the formation of sludge. And while oil suspends carbon, it’s also suspending dirt, ferrous metals and other contaminants that cannot be captured by filter media. Additionally, the longer you keep oil in the engine, the more the oil’s film strength decreases, providing less wear protection, even if very minimal. It does this dependent on the composition of the additive package (how well formulated it is), the temperature of the oil (how hot the engine is running), and the amount of contamination present in the oil. By the time you reach 5,000 miles, you could very well lose, in example, 50% film strength, which means 50% less wear protection.

All things considered, this breaks down oil change intervals into 3 schedules: 1) Regular Service; 2) Severe Service; and 3) Not Keeping the Vehicle for More Than 100-200K+ Miles, So Follow the Owner’s Manual. With that said, if you don’t intend to keep your vehicle more than 100-200K miles and you’re not an aggressive driver or racer, you need not be concerned with oil change intervals or this website at all. Just follow your owner’s manual, and highly dependent on how well your engine is designed, your car should last at minimum 100K miles, assuming you follow the owner’s manual. You may very well exceed 300K miles with the owner’s manual and even neglect of maintenance if you get a truly legendary engine. However, if you plan to actually keep any vehicle well over these numbers or want to keep it in excellent running condition for the next owner, the other two schedules definitely should be followed. Regardless of which schedule you follow, engine oil should always be replaced every 6 months if the scheduled miles are not met. This is because oil’s composition significantly degrades with time when contamination is present.

Engine Oil Change Schedule 1: Regular Service (Every 5,000 miles)

This is the schedule most people will be advised to follow by technicians, dealerships, and other advocates. It’s considered the proverbial magic number. But, if you fall into any of the conditions outlined in Schedule 2 (Severe Service), this schedule no longer applies to you. And what’s interesting is that the majority of drivers do not even qualify for Regular Service.

Engine Oil Change Schedule 2: Severe Service (Every 3,000 miles)

This is the schedule most people will likely need to follow. Yes, you read that correctly. The odds are you will likely need to adhere to Severe Service. If you meet ANY of the conditions below, you need to follow this schedule, without exception:

You take mostly short trips throughout any given week (5 miles or less) and you do not commute on the highway for at least 30 minutes in a single, non-stop trip and at a minimum speed of 55 miles per hour.
You drive with lots of stop-and-go traffic, city or highway.
You drive at low speeds only (less than 55 miles per hour).
You frequently idle your engine (10 minutes or more).
You drive in very hot or very cold weather.
You drive in dusty, sandy, salty, or muddy conditions, or you drive in highly polluted areas.
Your vehicle is usually weighted down (i.e., towing, loaded with passengers and/or items).
You frequently drive over rough terrain, including: steep hills, mountains, and gravel roads.

If you’re wondering why such conditions require Severe Service to be followed, here are some of the reasons:

Short trips don’t allow the engine to adequately raise operating engine temperature high enough for the oil to do its job and capture any carbon deposits that may exist within the engine. The inadequate temperature also doesn’t allow the oil to get hot enough to burn out any water within it from condensation; that is, inside of metal oil pans and other metal parts of the engine that don’t have oil present to protect them from water formation when the engine is off. When the engine is turned on, that condensation is washed off such surfaces and mixed in with the oil.
Idling your engine, driving at low speeds, and doing frequent stop and go driving especially increases oil dilution to occur. This happens normally but much more so in such circumstances. Oil dilution is when fuel gets past the pistons’ rings during combustion and into the oil supply. While the amount of fuel is usually relatively low, it’s also detrimental to the oil and especially the oil’s film strength. This is because fuel, even if a small amount, breaks down the composition of oil, essentially thinning it, thus, more wear is being caused to the engine.
Towing, fully loading your vehicle with passengers and supplies, and other forms of weighing it down have the engine (and transmission) work harder; this puts more strain on its components, increasing wear and shortening the additive package’s lifespan.
Driving in dusty, sandy, salty, muddy, or highly polluted conditions not only makes your air filter work harder and need to be replaced sooner, it also increases the number of contaminants that get past the filter’s media. These contaminants, namely dirt, enter into the combustion chamber and mix with the oil supply.

Simply put, oil is the absolute life blood of any gas or diesel-powered vehicle. Without it, the engine, transmission, and other gearboxes would destroy themselves. It’s spectacular how many people either neglect their oil or don’t take it all too seriously, and those same people often why their vehicles commonly fail only a few hundred thousand miles down the road, thinking that’s just how things are. Choosing a high quality oil and doing proper oil change intervals can make even faultily engineered engines and transmissions run much longer than usual.

Engine Oil Filter Change Intervals

After determining your proper oil change interval, you’ll next need to determine how often to change the engine oil filter. This is equally important as the oil itself, because without a good filter and appropriate change interval, your engine is vulnerable to wear damage. (As a side note, if you don’t want to have to also worry about when to change the filter, to be safe, you should change it every oil change to make things simple. Just be sure to use a high quality one. Going cheap on a filter is often not a good idea.)

So, here’s the deal with oil filters… There are now filters that claim 15,000-mile protection, some less and some more. Don’t believe the bullshit! Filters are cheap; engines are expensive! Overall, it’s a good rule of thumb to never go beyond 5,000 miles on an oil filter. The reason for this is simple: as the filter media becomes clogged with ferrous metals and other particles, it will eventually become obstructed, forcing oil to route through the bypass valve, which means you now have unfiltered oil circulating through your engine. This is very bad. Also, some oil filters don’t even include a bypass valve; therefore, when the filter media is completely obstructed on such filters, the engine becomes starved of oil and the engine begins tearing itself apart from almost no lubrication. This is the worst thing that could possibly happen. Always get an oil filter with a bypass valve!

Transmission Oil Change Intervals

Just as engine oil has many opinions on when it should be changed, so does transmission fluid. Moreover, transmission fluid works near identical to engine oil. The only difference is they have different additive packages. Thankfully, because transmission fluid is not abused nearly as much as engine oil, it doesn’t need to be changed anywhere near as often. Again, if you don’t intend to keep your vehicle for more than 100-200K miles, don’t worry about ever changing it.

The same principle of film strength applies to transmission fluid, although you’re not going to get carbon in your transmission because there is no combustion that occurs, also not introducing carbon contamination and fuel dilution.

Transmission Fluid Change Schedule 1: Regular Service (50,000 miles)

This is the schedule most people will be advised to follow by technicians, dealerships, and other advocates. It, too, is considered the proverbial magic number. But, if you fall into any of the conditions outlined in Schedule 2 (Severe Service), this schedule no longer applies to you.

Transmission Fluid Change Schedule 2: Severe Service (30,000 miles)

You drive with lots of stop-and-go traffic, city or highway.
You drive in very hot or very cold weather.
Your vehicle is usually weighted down (i.e., towing, loaded with passengers and/or items).
You frequently drive over rough terrain, including: steep hills, mountains, and gravel roads.

Transmission Oil Filter Change Intervals

Transmission oil filters are usually a lot different than engine oil filters; they typically filter >100 microns, whereas engine oil filters are >20-40 microns. However, this does not apply to all transmission filters! You need to verify by researching what filter your transmission is using. Some newer transmission filters are >20 microns. Therefore, the rule is if your transmission filter is >100 microns, you should never have to change it. Changing the fluid and cleaning out the drain pain and magnets of ferrous metals is enough to service it; however, if your filter is >20 / >30 / >40 microns, you should change the filter every time you service the transmission. Additionally, if you have a >100 micron filter and you ever notice your transmission has any delay in shifting, consider replacing the filter to see if that addresses your problem.

If you have a >100 micron filter or whatnot and you want added filtration to protect your transmission, consider installing an in-line filter to either the transmission inlet or outlet hose. Also, be certain such a filter is equipped with a bypass valve! Moreover, well engineered in-line filters typically have a low PSI threshold to actuate the bypass valve, so you shouldn’t have any problems with shifting when the filter media becomes obstructed. Change an in-line filter every transmission oil service or sooner.