Can you explain what the difference is between the AMSOIL SAE 5W-30 SYNTHETIC MOTOR OIL and Synthetic XL 5W-30 Motor Oil? The first one is slightly more expensive but has a recommended change interval of 25,000-miles/12-months, and the XL oil has a shorter change interval of up to 10,000-miles/6-months, but is there more to it then that?

I understand your confusion. Simply, both far exceed the specifications set for engines by API and SAE. The difference relates more to marketing than specifications. A few years ago, statistically for the first time, more people had someone else change their oil. For years, the do-it-yourselfers were more prevalent. When this happened AMSOIL decided to come out with an oil that would specifically address the market where people would have their oil changed at intervals up to 10,000 miles or 6-months, whichever came first. In addition, many modern vehicles now are equipped with oil change monitors. It is not unusual for these monitors to determine that the motor oil can remain servicable for even longer than 10,000 miles, some up to 12,000 miles. AMSOIL XL oils are formulated to maximize the oil change intervals in these situations and provide the maximum performance while providing maximum protection, and performance is not sacrificed with these longer drain intervals.

The Synthetic Signature Series 5W-30 and 10W-30 oil is a 25,000-mile/12-month oil. For this reason, it has a bit more intense additive package so that it will stay up to specification for the longer duration. If you are not using AMSOIL EaO oil filters, you need to change the oil filter with this oil at 12,500-miles/6-month. With an AMSOIL filter, one EaO filter will last up to 25,000 miles. The oil, however, is still a 25,000-mile/12-month oil. By the way, most all good quality oil filter will work with the XL oil as the better filters are made to go 7,500 miles or the length determined by the car manufacturers monitors.

A major change did come in AMSOIL as of July 1, 2002, with the XL oil. AMSOIL became somewhat uncompetitive as a result of changes to other oils in the "do-it-for-me" market. Oils that had previously been of similar non-petroleum base stocks, known in the industry as Group IV Base Stocks, switched to a petroleum base stock that was refined by a different process making hydroisomerized basestock, generally known as Group III Base Stocks. This improves the quality and performance of the oil, even though it is still a petroleum product, but has characteristics of a synthetic oil. Castrol was I believe the first, and the change drew a complaint from Mobil with the National Advertising Division (NAD) of the Council of Better Business Bureaus. Click Here to view the article on my newsletter page that gives the details.

The result was that "synthetic" is now a marketing term, not a definition rooted in the chemistry make-up of the lubricant. With the change of so many products to this technology (many oils on the shelf now carrying the label Synthetic is of this make up), AMSOIL, in order to remain competitive in this category of lubricants, changed the base stock of their XL oils (and only the XL oils) to a hydroisomerized basestock. This resulted in a cost reduction which AMSOIL passed on as a price reduction.

Generally, if you are concerned about getting the best performance, stick with the regular AMSOIL Synthetic Signature Series Oils. The XL is aimed at those looking at easing into extended drains.

XLT/Competitor Comparisons 10W-30 Motor Oils