What are the advantages of synthetics over petro based oils?
1. greater thermal stability, less thermal breakdown
2. greater detergency or cleaning ability
3. can flow at very low temperatures
4. higher vaporization temperature
5. less shearing under temperature and load
Thermal stability is important to long life of the lubricant. Additives and the base stock used are primary determiners of a lubricants thermal stability. Even if you see your oil temp guage show some normal reading it's important to know that in the engine there are much hotter areas for the oil to contend with than where the temp probe is located. Within the bearings, temperatures can approach 300 degrees F., and at the rings perhaps 500+ degrees. Petro based oil breakdown at these temps is assured.
Detergency is important to help sweep the combustion contaminants from the engine interior into the oil filter. Higher detergency prevents any buildup of these contaminants on the steel surfaces in the oil channels, and oil flow areas. Keeping these areas clear is important since any buildups can break free to contaminate smaller passages in the engine. This is probably why some people assert it's not a good idea to run a synthetic after using a petro based oil since the synthetics detergency is so great it might wash out prior contaminant deposits and foul the passages.
Low flow capability is an obvious advantage. Oil gets to where it needs to go qiucker during cold start conditions or in very cold weather. Synthetics can flow down to perhaps -40 F. Petro based oils with their parrafin wax dont do nearly as well in this regard.
Petro based oils have a tendency to vaporize off at high temperatures, which of course results in the deposits of contaminants that were in the oil prior to vaporization. These contaminant particles now have the ability to adhere to the interior surfaces of the engine.
Viscosity is everything in an engine. Petro based oils, once subject to high temperatures and high stress will thin out from the heat , and shear under increasing loads in the bearings. The hydrodynamic oil film in the sleeve bearing area of the mains is compromised. It will thin out, and the internal bearing oil pressure will be less. If there is a lesser thickness oil film in the bearing it will be able to handle lesser and lesser loads before metal to metal contact is made, and the soft main bearing material undermined. In a mains type sleeve bearing there is a very interesting oil flow shape, a resevoir of oil if you will, within the clearance of the bearing journal and bore. As best as I can describe it, it is a teardrop shape, and the oil is dragged off this teardrop by the crank journal into the high pressure region where the max load is. if this thin film boundary lube point is undermined, the engine is toast. Oil that is no longer up to carrying its rated load will not be able to prevent that metal to metal contact.
How hard are you on your oil? Not an easy question since not much is readily available to give you clear indicators how your driving style or trip lngths affect lubricants. Generally , if you dont let the oil and engine temps stabilize during a drive, thats hard on a lube. Trips over 10 miles should get you to those temps, trips under 10 miles or so, are rather hard on lubricants. Usually , on those short trips, combustion moisture and acids are not dealt with effectively by burning them off, nor have the piston rings fully temped up and seated. We all know what happens when the rings arent doing an effective job of sealing on an 11.5:1 compression engine.
Contrast the car that get 12,000 miles a year of highway trips of 30-150 miles duration with another car that gets 12,000 miles a year of 10 mile jaunts, easy to see now.
How about when you start the car? Do you let it get to temp befor you drive first, or do you take off within 30-60 seconds. Give yourself a star if you drive off within 30-60 seconds and dont let it idle for a long time. Idling a car to awrm her up takes far too long to get to operating temperature and is also very hard on the oil for the prior reasons also. Get that car moving, and keep the revs down low until shes all warmed up.
Now to get to the meat of the matter and see whats what with those Group IV and V synthetic lubricants.
IN a nutshell their manufactured from Polyalphaolefins, which are made from synthesizing hydrocarbons. They offer most but not all the benefits of a true synthetic. They lack in a true synthetics increased detergency and thermal stability, so they get some additive packages to to up thier performance in these areas. I do not refer to group IV based lubricants as true synthetics since they are still hydrocarbon based, even though man made. In this group , the additives usually give the different characteristics and performance differences in this group of lubricants. Amsoil and Mobil 1 are the two best known of the Group IV base stock lubricants. generally speaking, these oils will be appropriate for most peoples driving styles, and they should be changed at no greater intervals than perhaps 5000-6000 miles.
These are the true synthetics made from organic esters, not containing hydrocarbons at all. They have all the benefits listed at the top of this thread in abundance.
These oils usually do not require any additives, and that is the main reason they have such extended change intervals. Most oils need to be changed due to thier additive packs being depleted.
These oils are also very expensive, but a conservative change interval of 10,000 miles evens out the cost a bit. Some of us , myself included, change them out at 7500 miles just for the feel good factor.
By the way, even though you dont change the oil for extended miles, dont be confused , still change the filter at 3000 miles maximum. The synthetics deal with engine sludges and contaminants better than petro based oils, they do not prevent them from forming though, so you still have to filter them out.
I believe the best known brand name for this group is Redline.
The castrol we get from the BMW dealer is a Group IV based lubricant.
The Castrol Syntec you can buy at the local NAPPA ia Group III based oil.
group I - III based oils are just not applicable to our engines, forget about them.