If you own an older car, you might want to think twice about the oil you're using.

In order to comply with federal requirements that key emissions control components on new cars such as catalytic converters last at least 120,000 miles (previously, it was 100,000 miles) automakers have been pushing for reductions in an oil additive known as zinc dialkyl dithio phosphate (ZDDP), which contains phosphorous (as well as zinc and manganese).

The problem for late model emissions-controlled cars is that the phosphorous in ZDDP has been linked with premature catalytic converter failure - or at least, premature loss of converter efficiency.

But the problem for older cars with flat tappet camshafts - which means pretty much all cars built before about the mid-1980s, when roller camshafts began to supplant the flat tappet design - is that oils with low ZDDP levels can cause rapid premature wear, even failure, of flat tappet camshafts. In a nutshell, the ZDDP cushions the high pressure point between the lifter crown and the camshaft lobe, acting as anti-friction, anti-wear barrier.

Running without the ZDDP is almost like running without oil - and with the same results.

Levels of ZDDP in commonly available mainstream motor oils - including big-name brands and high dollar synthetics - have been dropping since the new emissions longevity requirements became effective with the 2004 model year. Unfortunately, many hobbyists and owners of older cars with flat tappet camshafts are unaware of the changing formulations - and the threat low-ZDDP oils may represent.

The situation is analogous to the days when lead began to disappear from gasoline. Engines that had been designed to burn leaded fuel (especially high-performance engines run at high RPMs) fell victim to premature valve recession caused by the use of unleaded fuel.

What to do?

The first thing is to determine whether your vehicle is equipped with a flat tappet camshaft. If it's an American-brand car older than model year 1980 and the engine is either original or has been rebuilt to original specifications, the odds are virtually 100 percent certain that you have a flat tappet camshaft. It's also very likely you have one if your car is early-mid 1980s. By the latter half of the '80s and into the 1990s, roller-style camshafts were becoming the norm - and you are probably safe. But it's important to be sure. You won't find information on the type of camshaft your vehicle has in your owner's manual. You'll need to consult a technical service manual - or simply ask someone who is knowledgeable. The service manager at a dealership for your make/model of car ought to know - or should be able to find out.

What to use?

There are still a few oils on the market that have adequate levels of ZDDP. These include Shell Rotella T - a conventional (mineral-based) oil that was originally formulated for diesel engines. Rotella T still contains 1,200 parts per million ZDDP, according to Shell - which is as much as five times the amount found in other oils. Don't sweat it that Rotella was/is "for diesels." It's also an excellent choice for older, non-emissions controlled engines with flat tappet cams that need their ZDDP. Rotella's also modestly priced and readily available at most any auto parts store. Shell also markets a synthetic version of Rotella that offers even more protection - as well as longevity and a 5W-40 viscosity for those who operate their vehicles in colder climates. Standard Rotella comes in a heavier 15W-40 blend.

Another choice - in a full synthetic - is Amsoil (www.amsoil.com) which carries a line of oils with ZDDP in popular viscosities such as 10W-40 and heavier 20W-50. Redline oil (www.redlineoil.com) is also still fine for older engines with flat tappet cams. Unfortunately, both Amsoil and Redline can be hard to find at your local store; but if you plan ahead, you can order a case from any one of multiple suppliers online and just keep a stash on hand.

Another option is additives. GM used to sell an over the counter Engine Oil Supplement (EOS) that was just what the doctor ordered - and for only about $12 per bottle. Unfortunately, GM stopped making the stuff and it's now very hard to find.

Luckily, Competition Cams does offer something similar - its Engine Break-in Oil Additive. Comp cams used to recommend this for initial break-in but now recommends that it be added with the oil at every oil change. Here's the skinny from Tech Bulletin 225:

"While this additive was originally developed specifically for break-in protection, subsequent testing has proven the durability benefits of its long term use. This special blend of additives promotes proper break-in and protects against premature cam and lifter failure by replacing some of the beneficial ingredients that the oil companies have been required to remove from off the shelf oil."

Here's the link to Comp Cams and the page for the additive they sell: http://www.compperformancegroupstore...Category_Code=

So there you have it.

If you own an older vehicle, you'd be well-advised to give some thought to your next oil change - and what kind of oil you'll be pouring into the crankcase.