Determining oil change intervals via analysis

John's post on Valvoline's oil change interval survey results got the attention of our readers, so perhaps now it's time for a slightly different perspective on the topic. If one accepts the fact that each combination of driver and engine is likely to require its own oil change interval, then the idea of some sort of "blanket" recommendation on the topic seems almost silly.

With this in mind, we decided to run some analysis on a couple of vehicles in our motor pool. Due to the mileage put on our 2001 Honda Accord V6 during its daily commute, the 3,000-mile interval suggested by ASE Master Mechanics would require a change every month and a half. The service manual, on the other hand, suggests that the oil need be changed only every 8,000 miles - an interval a bit more practical for busy DIYers. A couple simple and affordable tests from Blackstone Labs offered some insight on who might be right. And what sort of stress does the high-revving stroker V8 in our Impala SS impart upon its oil? Click through to find out.

The sampling process is simple enough - just use one of Blackstone's free kits to grab a few ounces of oil during your next change. It's best that the sample be taken from somewhere in the middle of the drain process, and obviously a bit of caution is required if doing this if the engine has been recently run. Next, fill out the necessary information on the included paperwork, including the make and model of the car, the type of engine, the brand and weight of oil, and the number of miles put on the vehicle since the previous oil change. Include the necessary payment ($30), and drop it in a mailbox. Blackstone analyzes the oil using inductive coupled plasma (ICP) spectrometry, and in about a week, the results will show up in your inbox (or snailmail box) looking much like this [click on the photo for a larger version]:

You can see here that results will show the levels of a variety of elements (additives and contaminants), some physical properties of the sample, and some commentary from the lab. Additionally, the element levels from previous samples are shown, as well as averages from that testing, and averages from other users who have the exact same type of engine.

In this particular case, we're looking for any signs of excess wear, which would come in the form of elevated levels of iron, aluminum, lead, and copper. While the sample from 5/1/05 shows some wear results that are slightly above average, the results from the 9/11/05 test are actually a bit less than average, even after 8,700 miles of usage. Additionally, the Total Base Number, or TBN (an indication of the oil's ability to neutralize acid) is low but still safe at 2.1. The subviscosity and flashpoint were also within the acceptable range for this oil viscosity. While Blackstone states that it may be possible to run this oil change out to 9,500 miles, we'd probably consider the factory recommendation of 8,000 miles to be just about right.

Castrol GTX 5W20 was used for the first test, while Valvoline 5W20 was used for the second round of analysis. This doesn't necessarily mean that the latter should be called a better oil, but it does seem better suited for this particular engine.

Obviously, each engine, driver, and environment will have different effects, and that doesn't even include the variables that we can add with all the different oils and filters that are available on the market. Some experimentation will be required to determine the optimum change interval for any particular car, and it's always going to be better to err on the side of caution until sufficient data has been accumulated.

So, what about our resident hot rod?

We run Castrol 20W50 in this engine to give it just a bit of extra protection at high revs and temps, and yet the subviscosity was below the expected value after only 2,600 miles. We've been trying to maintain a change interval of about 2,500 miles on this engine (which works out to be once a year), and that seems about right.

Due to the relative newness of this engine, we're not yet worried about the high levels of iron, but we will be looking for that to decrease over time. The copper and lead look good, so we're not in the process of wiping any bearings. One can also note the high level of silicon in this sample, although this is likely explained not by contamination by dirt but rather the use of silicone gaskets and RTV silicone sealer that was applied during its most recent reassembly. We were definitely happy no fuel dilution had occurred, despite the fact that this engine tends to run a bit rich while warming-up.

Hopefully we've demonstrated the value of oil analysis. We certainly regard it as a good value, and will continue to utilize it on the large variety of engines that we maintain here at the Autoblog Garage.