Perhaps silly, but Synthetic Oil weight correlate exactly to convention oil weights ?

A very common situation where I live. People take their vehicles up into the Sierra Nevada and sub-freezing starts are routine.

Good point - I've never seen conventional 5W-40. I was trying to make a point without thinking it through ;-^

This is categorically NOT true for Mobil 1 synthetic. 10W-30 is not as likely to slip past the rings in a cold engine with high mileage and moderate to high engine wear. The disadvantage compared to 5W-30 is that the later provides better fuel economy (at least for an engine without excessive wear).

Synthetic oil is a completely different beast than conventional oil, and there is not a lot of difference in terms of "protection" between the different viscosities of the same brand. Here is a comparison on Mobil 1

5W-30 and 0W-40 (Mobil 1 does not have a 10W-40).

Notice that the pour points are the same, and the flash points are nearly the same (230 C versus 236 C). 230 Celsius is 446 degrees Fahrenheit, which is way beyond the normal operating temperature of a modern water cooled automobile engine (most fully warm engines run at about 90 degrees Celsius or about 194 degrees F).

In the old days, using a conventional oil with a higher viscosity (and higher flashpoint) was important because the oil tended to break down and loose viscosity after a short time. But this is not true of synthetic oils, so there is no real advantage to using a higher viscosity than necessary in order to provide proper engine protection. What is important is to provide the right viscosity oil for a specific engine, which can vary over time if there is a lot of engine wear (such as would happen with using conventional oil).

Mobil 1 5W-30

SAE Grade 5W-30 Viscosity, ASTM D 445 cSt @ 40º C 64.8 cSt @ 100º C 11.3 Viscosity Index, ASTM D 2270 169 Sulfated Ash, wt%, ASTM D 874 1.0 HTHS Viscosity, mPa·s @ 150ºC ASTM D 4683 3.09 Pour Point, ºC, ASTM D 97 -54 Flash Point, ºC, ASTM D 92 230 Density @15º C kg/l, ASTM D 4052 0.80

Mobil 1 0W-40

SAE Grade 0W-40 Viscosity, ASTM D 445 cSt @ 40º C 80 cSt @ 100º C 14.3 Viscosity Index, ASTM D 2270 187 Sulfated Ash, wt%, ASTM D 874 1.2 HTHS Viscosity, mPa·s @ 150ºC ASTM D 4683 3.6 Pour Point, ºC, ASTM D 97 -54 Flash Point, ºC, ASTM D 92 236 Density @15º C kg/l, ASTM D 4052 0.855

Thanks for the update, Ray. The website indicates that not too much advantage is to be had from synthetics, however everyone I know seems to think that synthetics are decidedly preferable. In any event, over to Mobil 1 at 5K it is .

You're welcome!

Synthetic oil makers used to claim that it was safe to extend oil change intervals, but the problem with that claim is that they would be on the hook in the event of engine damage instead of the automaker, so the advantage of extended change intervals is pretty much gone now. I do believe that synthetic oil performs better than conventional oil, but I also look at things from a cost - benefit point of view. If the engine is normally aspirated (no turbo or supercharger) and you are only going to keep the vehicle for a relatively short period of time (say < 100,000 miles) and follow factory recommended oil change intervals, then you probably will not recover the additional cost of the oil when you sell the vehicle. On the other hand, if you are going to keep the vehicle until it dies, then there is a benefit from synthetics if the engine lasts 100,000 miles or whatever longer than it would have with conventional oil and the rest of the vehicle holds together as well. There is the argument that documentation of the use of synthetic oil will enhance the value of the vehicle at trade-in, but I've never seen an "add" in the NADA guide, KBB, Edmunds, etc. to the value of the vehicle for synthetic use.

I do not advocate or discourage the use of synthetic oil; I just try to get people a clearer picture of the costs and benefits.

I would love to see evidence of this. Everything I've read indicates that for "normal" engines (non-turbo) operated in moderate climates, that there is no benefit to synthetic, unless the owner plans to extend the oil change interval (at least after the warranty period).

Synthetic has a benefit in very cold climates, and for turbo-charged engines, and if you're going to extend the oil change interval.

Above all, use an API certified synthetic, there are some bad synthetics out there, offered by multi-level marketing companies.

I have not seen any evidence from tests or studies conducted under controlled conditions that an engine will last 100,000 miles longer with synthetic either, which is which is why I said "..there is a benefit IF the engine lasts 100,000 miles or longer..."

But the factors which make an engine last that long go far beyond synthetic versus petroleum based oils. Both products are highly engineered products, other than the difference in the source of the base stock. A Toyota engine will easily go 200K+ miles with either type of oil, provided that there is not some catastrophic failure that is unrelated to the oil, such as a timing belt failure on an interference engine, excessive overheating, or a loss of engine oil.

Certainly the wear tests that have been done that measured engine wear with synthetic versus petroleum based oils showed no difference in wear. Nor for that matter were there any difference in wear between the different oil change intervals.

It sounds like you are arguing a point with me, which I don't understand because we seem to agree in principle. ;-)

If one changes their oil at 3000 intervals (as many do), then using synthetic oil at 6000 mile intervals will be about the same cost, and the synthetic oil will provide better engine protection at 6000 miles than the conventional oil does at 3000 miles. So on that basis, the cost differential may be non-existent or minimal.

In addition, a 5W-30 synthetic oil provides better fuel economy than conventional 5W-30 oil. Not much better, but at least several dollars worth at every oil change. A .1/4 gallon per mile increase (20.25 vs. 20 MPG) in gas mileage will yield about $10 savings every 6000 miles.

This is true, but my example assumed that the oil change intervals would be

5,000 miles in both cases. Toyota specifically warns not to extend the oil change intervals beyone their recommendations with synthetic oils. Mobil does provide some engine guarantee with Mobil 1, and if their coverage is as good as the factory warranty, then there may be merit to going past the factory recommended intervals.

I am not anti-synthetic. If one wants the best motor oil available, then synthetic is the way to go. All I'm saying is that if one follows the factory recommended oil change intervals and trades vehicles every 3 to 5 years, then conventional oil is certainly good enough.

I don't know about the newer cars, but my 98 Camry specifies 7500 between oil changes (5000 miles for severe service). I feel very comfortable in going 6000 miles with Mobil 1 5W-30. I would not feel comfortable going 6000 miles with conventional oil. Therefore, for me at least (and maybe others) the cost of a full synthetic is not much higher (if any) than using conventional oil.

The upside of a full synthetic is not having to worry about gelling, and not worrying as much about worn valves and other problems associated with higher mileage engines.

for the OP's 2006 4Runner, the factory recommended oil change interval is

5,000 miles.

Then in that case, one can go from 3000 mile intervals with conventional oil (used by many people) to 5000 mile intervals with synthetic oil. This would still make the price of a synthetic oil change reasonably close to the price of a conventional oil change. When you consider the other benefits, like increased fuel mileage, then it gets even closer to the same cost.

I fairly certain that if synthetic oil was required and used by everyone, the oil change interval recommended by Toyota would be higher than 5000 miles.