A reader asks: I have a 2006 Honda S2000 that is driven just 1000 miles per year. The few miles that it does see are very easy miles. I last changed the rear differential gear oil about 4 years and 5000 miles ago. The car is stored in a warm garage and the diff has never been exposed to moisture. The gear oil used was AMSOIL Severe Gear 75W-90. How often should I change this gear oil when the car sees so few miles?

Our answer: Many car enthusiasts own have vehicles that they affectionately refer to as a “garage queen”. While these cars may see very few miles, maintenance still has to be a consideration.

When it comes to gear oil service intervals in sparsely driven cars, there isn’t much consensus. As differential oil that isn’t exposed to moisture, extreme heat or heavy loads has a very easy life.

That being said, everything oxidizes over time and eventually the additive package of the gear oil will undergo “fallout”. A very conservative service interval would be every 5 to 7 years. We are confident that a robust product like Severe Gear would last much longer. But this is likely a good window to freshen up the gear oil. This will keep corrosion at bay, wear control at its best and seals supple.

Other Stored Car Maintenance Notes

Regular maintenance is still required for cars that are rarely driven. Here are some notes on the subject.

  • Fuel degradation is a massive concern and should be considered at all times. Make sure to add gasoline stabilizer when filling the tank. Degraded fuel can foul carburetors or injectors and fuel lines. Bad gasoline can also cause depositing and significant engine performance issues.
  • Still on fuel issues, it’s a good idea to add carburetor or injector cleaner to the gas tank at least once per year.
  • Change the engine oil at least once per year. This is the case regardless of how few miles the car travels over a year. Combustion by-products allow for acid formation in the oil over time. Older vehicles also have a higher tendency toward fuel dilution in the oil. Changing the motor oil just before long-term storage is the best course of action.
  • Brake fluid tends to draw moisture out of the air, even though it is a sealed system. Brake fluid should be changed slightly more often than rear differential gear oil (every 4-5 years or so). Fresh brake fluid prevents the creation of rust and corrosion in the braking system.

AMSOIL OE 0W-16 Synthetic Motor OilA reader asks: Will AMSOIL be coming out with a Signature Series 0W-16 oil anytime soon?

Our answer: It will almost certainly happen, but we wouldn’t expect it in the immediate future. For now, just the Honda Fit and certain Toyota Camry models take 0W-16 motor oil. So it goes without saying that the market for 0W-16 motor is currently very tiny. As of now, AMSOIL offers the value-packed OE 0W-16 Synthetic Motor Oil.

We expect that many more automakers will start to suggest 0W-16 for some models in the next few years. This would likely prompt AMSOIL to expand the 0W-16 offerings to the higher-end Signature Series and XL Series.

If auto manufacturers make a significant move to 0W-16 motor oil in 2021-22 (and that is plausible), it would expand the 0W-16 market overnight. But again, until more engines are designed for 0W-16 motor oil, we would expect that AMSOIL will offer only the OE Series version.

AMSOIL Signature Series 15W-40 Max-Duty Synthetic Diesel OilA reader asks: I’ve got a 2003 Dodge Ram 2500 with the 5.9L Cummins engine. Should I use the older generation of CI-4+ diesel oils? Are the newer API CK-4 diesel oils good for this engine? My understanding is that these engines have flat tappet cams and I hear that the newer diesel oils do not have enough zinc to protect them.

Our answer: The short answer is that (for your older engine), virtually any of our diesel motor oils will be highly effective. The long answer may surprise you.

There is a common misconception that the newest diesel motor oils meeting the latest API CK-4 specification are somehow weaker or less robust than previous generations of diesel oils. This is patently false.

In fact, API CK-4 diesel oils are considerably more sophisticated and capable than diesel oils meeting previous specifications such API CI-4+. Older diesel oil formulas meeting API CI-4+ are weaker in terms of protective capabilities.

The introduction of the CK-4 spec was a tremendous challenge for motor oil makers. Diesel engines and their emissions components have become very sophisticated in recent years. The CK-4 requirements required oil makers to ramp up their game to meet much higher standards. Here are some of the upgrades of the API CK-4 spec over the previous CJ-4 spec.

  • Increase in high temperature capabilities
  • Improvement of piston and bore wear protection
  • Better deposit control
  • Resistance to degradation of hot and cold weather properties
  • Improved shear stability
  • Enhanced aeration resistance
  • Resistance to catalyst and particulate filter fouling
  • Improved oxidation resistance

What About AMSOIL Heavy Duty Series Vs. Older Diesel Oils?

AMSOIL Heavy Duty Diesel Series is a less expensive API CK-4 option to the flagship Signature Series Max-Duty Diesel Series. Heavy Duty Series was created as a budget-friendly option for fleets, the oil change installer market and those looking for a less expensive synthetic product.

We asked AMSOIL Technical Services how the API CK-4 AMSOIL Heavy Duty 15W-40 might compare to an older generation product like the API CI-4+ “AME” AMSOIL Heavy Duty Diesel and Marine 15W-40. Their answer was “It’s not even close”. While the Heavy-Duty Diesel And Marine 15W-40 is a very robust and competent product, development stopped when the API CI-4+ specification was replaced in 2007. The ball has been moved way down the field since then. So even the less expensive API CK-4 diesel oils are a step up over the previous generation.

Signature Series Max-Duty is considerably more advanced. It offers the most advanced synthetic base oil and additive technology available. As a result, Signature Series Max-Duty is an upgrade over Heavy-Duty Series in terms of wear control and service life.

What About Flat Tappet Cams?

Flat tappet cams are relatively common to many diesel engine types. The biggest concern with flat tappet cams is wear protection. Specifically, does the motor oil contain enough ZDDP (Zinc Dialkyl Dithio Phosphate) and phosphorus?

Flat tappets in diesel engines are less of a concern than having flat tappet cams in something like a gasoline-powered street rod motor. Street rods and drag racers run at much higher RPM and tend to have much heavier valve springs compared to stock diesel engines. Also, modern passenger car/gasoline motor oils have much lower zinc and phosphorous limits. These levels are insufficient for flat tappet wear protection. So owners of gasoline engines with flat tappet camshafts have to be very mindful of their motor oil choice.

This is not a concern with diesel engine oil. Diesel motor oils are designed for all diesel engine types. Contrary to popular belief, the newest API CK-4 spec has not limited zinc and phosphorous to dangerously low levels with 40-weight viscosities. This would include 0W-40, 5W-40 and 15W-40, etc.

The API CK-4 AMSOIL Signature Series Max-Duty 5W-40 and 15W-40 has zinc levels over 1300 and phosphorus levels above 1200 ppm. Similarly, the AMSOIL Heavy- Duty 5W-40 and 15W-40 has zinc over 1100 ppm and phosphorus over 1000 ppm.

To contrast, the older AMSOIL Heavy Duty Diesel and Marine 15W-40 (API CI-4+) has 1377 PPM ZDDP and 1267 ppm phosphorus. This doesn’t necessarily mean that it will protect flat tappet engines better than the API CK-4 oils. The zinc and phosphorus level comparisons are a moot point. The newer API CK-4 chemistry is miles ahead of the older API CI-4+ formula. The newer CK-4 options will provide considerably better wear protection.


To sum up, do not be concerned about using the latest API CK-4 diesel motor oils in older engines. They are very much backward compatible with older engine designs and are a giant step forward in terms of wear control.