1999-2009 [Subscribe to Daily Digest] |
First of all, most of the widely sold synthetic brands are Group III base oil - hydroisomerized natural product. Most conventional oils now are either Group II (hydroisomerized natural product, only not as much) or a mixture of Group I (distilled crude) and group III or IV (Polyalfaolefin). So it's sort of the same stuff. All name brand motor oil has "seal sweller and conditioner" additives, and your car is new enough to have materials that are compatible with all oils on the market today. So they say.
That said, Ms. Jessica's Red Car (C900T) didn't leak a drop until I poured in the Rotella T synthetic that I've always used in my 9-5, and it immediately gushed out the front of the engine. (the part that's closest to the back of the car) Ms. Jessica's red car had 180,000 miles at the time, with an uncertain past.
But I still recommend synthetic, and the reason for using synthetic isn't to have longer change intervals.
Conventional multigrade oil is a light weight base oil combined with viscosity modifiers - very long molecules that curl up into a tiny ball when cold, but relax and spread out when warm. That's fine, but when squeezed and stretched - what happens in bearings, they get all strung out and then don't work as well. So your 10W30 motor oil, which looks like a 30 weight when the oil pump and rings and things see it, resorts toward being a 10 weight when it gets into the bearings - maybe not all the way, but more like a 20 weight or 15 weight than 30. Wait - it gets worse. To reduce sludge, Saab runs the oil some 40 degrees (F) hotter than they used to in the days of the C900, which is the same as taking two steps down on the viscosity scale. Now your 10W30 looks more like a "5" weight oil than a 30 weight oil. Add 90,000 miles of wear into the equation and you may be treading on thin ice - or thin film, to be more precise.
Synthetic oil is an inherently multi-grade liquid. Even without any viscosity modifiers, it might measure out as a 5W30, and that "30" will hold true at all shear rates. It's what's called a "newtonian" fluid. Water is a newtonian fluid - same viscosity no matter how high the shear rate. Natural oil - the pure base stock - is newtonian, and essentialy a single grade. Sure, they usually put some viscosity modifiers in synthetic to widen the range, but not nearly so much as with conventional oil.
Shell Rotella T synthetic 5W40 has the same viscosity under high shear rates as a natural "40" weight (something like 4.2, to give it a number). Compare that to Mobil 1 0W40. A synthetic oil, but made with a thinner base stock and with more viscosity index modifers, it behaves almost like a 30 weight under high shear. To give it a number, it measures 3.6. Castrol 0W30 mesures 3.5. There has to be a line somewhere between 0W30 and 0W40, and the Mobil and Castrol products couldn't be closer to that line - Castrol on the thinner (30) side, Mobil on the heavier side. Both the Mobil and Castrol that I mention here meet GM's long life spec for gasoline engines, GM LL 25A/B, as required when following to the oil condition monitor in the 9-3SS. But I wouldn't (and don't) use either one in a 9-5 4 cylinder, on account of the higher oil temperature. (by comparison, the 9-3 oil uses the engine coolant to control its temperature - 90C, according to the thermostat. The oil thermostat in the air-to-oil 9-5 is set for something over 100C, the difference of about one full viscosity grade). There's a number of 5W40 synthetic oils out there. Castrol makes one. Mobil labels theirs "Turbo-diesel", and there's the Shell, which is the one I have been using.
As an alternative to the full synthetic, you could also use a 10W40 synthetic blend, or a 15W40 natural oil, and achieve the same high temperature lubrication. Neither one will flow as well at startup, with increasingly poor low temperature flow as the weather gets colder, so it kind of depends on where you live. Live in Flordia? You can safely use a 15W oil. On a crisp, wintry 60 degree Florida morning you will have about the same cold-start lubrication with 15W oil as your Minnesota bretheren have with their 5W oil during the occasional 30 degree "warm spell"
Hope this helps.
posted by 24.166.95...
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