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C5 Oil Changes

A year and a half ago, while getting ready to service my first C5 (I do all my own oil changes), I was shocked to find that the oil GM recommends (Mobil 1 5W30) not only costs up to $4.50 a quart, but that the car holds an unheard of amount of oil. In other words, I wasn't too happy about spending $50 on oil! I did some shopping around, and happily discovered there was an alternative. At that time, the only other oil I could find that met the required GM 4718 standard was Havoline Synthetic 5W30. And guess what, it only cost me $2.99 a quart, plus there was a $1.00 per quart rebate at that time. OK, do I spend $1.99 for the Havoline, or $4.50 for the Mobil 1. I think you can guess the answer. Well, now that some time has passed, I decided to re-address the Corvette oil dilemma, and see what help I could be to the others who might be facing it.

Let me start by talking a little about oil, viscosity, and what oil does in your car. Oil acts mainly as a lubricant in the LS-1 engine, but also acts as a coolant source for the rotating assemblies (antifreeze gets nowhere near these parts). Lubrication is a little more complicated than you might realize, however. In the journal of a rotating shaft (camshaft, crankshaft, rod journal), there is a thin layer of oil that, when pressurized, creates an impregnable boundary between the journal and the bearing. In other words, when you have oil pressure, you have NO METAL-TO-METAL-CONTACT between parts (in theory, of course). This is why the folks who make the engine treatments talk about start-up wear all of the time, because that is when the parts scrape against each other. Once you have good oil pressure, very little wear can occur in your engine!

This leads me to the oil itself. Oil is a viscous fluid that, as you probably know, gets less viscous as it gets warmer. This creates a problem. In the old days, when you started your car, the oil was around 10 times thicker than when the car reached operating temperatures. Today, with the help of multi-weight oils, and even more so with synthetic oils, this isn't the case. Today, an oil that meets 5W30 standards, will get thinner as it warms, but nearly as much as a straight 30W oil (Actually, I guess I should say that the 5W30 does not get as thick as the 30W when it cools). In case you don't know, 5W30 means it acts like a 5W oil when cold, and a 30W oil when hot (see FIG 1). An ideal oil would stay the same viscosity at all temperatures, but for now we must be happy that the oil companies have made the strides that they have. As for viscosity, it is simply a number (measure in centipoise) that represents a fluids shear strain against parts sliding around it (like a journal bearing). In other words, more viscosity equals more friction, and less flow.

So in order to protect you engine, GM had to find an oil that maintained adequate oil viscosity at running temperatures to protect the engine, while being thin enough at low temperature to lubricate the engine quickly upon start-up. This is the reason that 5W30 oil is suggested for almost all new cars today. Compared to other weight oils, this is the best compromise available. For the Corvette, however, that was not enough! GM feels that due to the performance nature of the vehicle, high oil temperatures could become a problem due to the lack of an oil cooler on the Corvette. Over time, synthetic oils maintain their viscosity better than conventional oils. Hence, Mobil 1 met both criteria!

Unfortunately, I feel Mobil 1 is taking advantage of customers by price gauging their product! I find it hard to believe that is costs 3-4 times as much to manufacture their synthetic oil than conventional oils. So I decided to find out! I have been testing several oils in an attemt to find a suitable substitute for Mobil 1. I like Mobil 1, and as you can see from the charts, it is a great product. However, other companies offer similar products, and for less money.

In my area, as of August 99', the following oils are readily available and meet the GM 4718 requirements for the Corvette Mobil 1 ($3.99-$4.49)), Havoline Synthetic ($2.99), Valvoline Synpower ($3.99), and Castrol Syntec ($4.29). I decided to test the oils to make sure they are similar to the Mobil 1 standard GM has chosen, and to see how they differ. I also tested Mobil 1 0W30 and 15W50 as possible alternatives for those of us who use our cars either in cold climates, or in hot climates or on track.

The charts say it all (FIG 2). All of the synthetic 5W30 oils are similar to the Mobil 1 standard. The Syntec is the closest (nearly identical), but costs about the same! The Havoline is slightly thicker at cooler temperatures, but for 30% less cost, I can live with it. It never gets cold enough here in NC to matter, anyway. The worst of the bunch (still very good) was the Valvoline. Again, nearly identical at higher temperatures, but more viscous at start-up. Valvoline was cheaper than the Mobil 1, but I am not sure it saves you enough to warrant the change. Obviously, my choice is the Havoline Synthetic. It costs nearly $1.50 cheaper per bottle, and offers great performance with only slight low temperature sacrifice. Just be sure to warm your car properly before revving it up too much!

For extreme conditions, I tested the Mobil 1 0W30 and 15W50. As you can see from the final chart (FIG 3), the 0W30 oil is very, very similar to the 5W30 oils listed above. However, at colder temperatures (start-up in winter) this oil offers a slight flow advantage over them. In fact, the oil is so similar at higher temperatures, I am not sure why GM doesn't recommend it instead of the 5W30 for all the Corvette!

To look at the 15W50, we must first talk a little about temperature. In daily driving, most cars' oil temperature may reach 200-210 degrees Fahrenheit. In other words, your oil might get as thin as a viscosity in the low teens (centipoise). On track or other severe circumstances, oil temperatures often reach around 250-290 degrees. By this temperature, 5W30 oil has gotten significantly thinner than normal. That can be very dangerous as oil can slosh around in your engine, or even worse, could weaken the fluid boundary protecting the moving parts in your engine! But guess what, Mobil 1 saves the day (FIG 4)! Their 15W50 oil at the higher temperatures has viscosity very similar to the 5W30 at normal temperatures. At this time, I have seen no other oil on the market that can offer this viscosity and meet the GM 4718 requirements. One note of warning thought, at start-up, the 15W50 is much thicker than the 5W30! Be careful if you are using this oil, and warm your car up well before driving it hard!

You can draw your own conclusions from this information. I feel all of the available synthetic oils would be safe in your cars. Mobil 1 makes a great product, and in the extreme cases, possibly your only choice. However, for regular driving and the occasional high speed pass, I think you have some options! Take a look around your area and see which of these oils are available. And if you find more that meet the GM 4718, send me a bottle and I'll throw it in the viscometer! Happy Driving!

David Farmer

Albemarle NC


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