If you ever want to start an argument, just tout a particular brand of oil to a group of automotive enthusiasts and sit back to watch the fireworks. Other than perhaps choice of tires, nothing seems to be as personal about one's car as the choice of oil for the engine. The debate rages on as to whether synthetic oil's superior (if indeed people can agree synthetic oil is superior) properties but dear price is worth it compared to conventional or "dino" oil. Let's try to separate fact from fiction and get to the bottom of this admittedly slippery debate.
Oil must accomplish two main tasks (and many smaller ones) in it's life within the engine: Lubricating the moving parts to reduce wear, and carrying away excess heat. Additionally, oil must not foam, it must not break down under extreme pressures, it must flow at cold temps yet not thin out at high temperatures, and it must neutralize acids and bases that tend to accumulate in the engine over time. Clearly an almost impossible task, yet oil accomplishes this miracle daily.
If you want to know what is right for your car, begin with the Owner's Manual where you will find a chart of approved oils and weight ranges. Study it and make your decision based on the ambient temperatures expected until the next oil change. The chart will list oils such as 10W-30, 10W-40, etc. The W in 10W-30 stands for Winter rather than Weight, because the oil has been approved for use during the winter months. The numbers refer to the weight range of the oil. When cold, a 10W-30 oil will act like a 10 weight, but when hot it will act like a thicker 30 weight. The current quality standard for oils is a rating of SJ which supercedes all lower ratings, such as SF, SG, SH etc. for gasoline engines. Mercedes used to specify certain brands of oil that met their rigid standards, but changed and now approve if the oil meets the current standards, regardless of brand name. Diesels, by virtue of their unique fuel source and content, pose additional stresses on oils; hence, a separate rating for approved diesel oils; CD, CE, CF, etc. The C in this case refers to Compression ignition, while the S for gas engines stands for Spark ignition.
A typical quart of "dino" oil costs about $1.25 while a quart of synthetic is about $4.00. What's so special about synthetic to make it soooo expensive? Synthetic oil is a bit of a misnomer in the sense that the base stock from which it is derived consists of highly purified and uniform "dino" oil molecules. Most synthetic oil manufacturers start with similar base stock but then add their own packages of additives that they think will result in superior engine performance. Does synthetic work? Yes, yes, yes! Some synthetic oils remain a liquid down to -60 F, which is far lower than regular oil, yet they continue to maintain their film thickness at high temperatures, which greatly reduces wear. Diesel owners take heed: In extreme cold, synthetic oil will allow the engine to turn over faster and may be just the difference between starting and being stranded. There are in fact synthetic oils made just for diesels, but may be difficult to find. Two examples are Redline and Delvac.
The rap against using synthetic oil, other than the obvious 3X cost differential, has been the question of leakage. When synthetic oil was first introduced, manufacturers did not get the additive packages right, and oil leaked through seals and gaskets that did not swell as they normally would with conventional oil. Over time, the additive packages have improved and reduced leaking. It is safe to say that if you have an oil leak with regular oil you will have at least as much of an oil leak with synthetic and possibly more. It's just slipperier.
Does DaimlerChrysler approve the use of synthetic oil for Mercedes cars? I am not authorized to speak for DaimlerChrysler, but unless the Owner's Manual specifically forbids synthetic oil, then tacit approval is implied as long as the oil meets or beats all specifications in the Manual. I might also add that DaimlerChrysler now REQUIRES all MBs with the FSS system (starting in 1998) to use synthetic oil ONLY.
The obvious question is whether oil change intervals can be extended in order to recoup some of the extra cost of synthetic oil. Under no circumstances is it approved to go beyond the changing interval as specified in the Owner's Manual (the new FSS notwithstanding); however, if your oil is changed more frequently, only YOU can decide when is best to do an oil change (always change the filter too). In the end, as the debate rages, each owner must weigh his or her own individual circumstance and decide what is best. Time for me to slip-slide out of here.
Steven Rae, Technical Advisor