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Motor Oil 103

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Motor Oil 103
Part Three. You have a synthetic mind.

Let us compare mineral and synthetic oils. I will not talk about chemical but rather functional differences. We discussed before how mineral oils are too thick at startup yet too thin when hot. The viscosity was corrected with the hot engine by adding VI improvers.

A 10W-30 multigrade mineral based oil is made from a 10 weight oil and has VI improvers added to thicken the product in a 212 F engine. It acts as a 30 weight oil when hot. It acts more as a 10 weight oil at startup. I remind you that a 10 or 5 or 2 weight oil is still too thick to provide lubrication at startup. They are all too thick at startup. There is currently no engine oil thin enough to operate correctly at startup. They all cause excessive wear at startup. Again, we are discussing the needs of my single hypothetical engine for around town driving.

Oil type.. Thickness at 75 F ..Thickness at 212 F

Straight 30..........250....................10

Straight 10..........30.....................6
Straight 5...........20.....................4
Straight 2...........15.....................3
Straight 0...........12.....................3 est.

A 10W-30 synthetic oil is based on a 30 weight oil. This is unlike the counterpart mineral oil based on a 10 weight oil. There is no VI improver needed. The oil is already correct for the normal operating temperature of 212 F. It has a thickness of 10 while you drive to work. It will never thin yet has the same long term problem as the mineral based oil. They both thicken with extended age.

Synthetic oils are derived in the laboratory. They are pure, usually nearly clear. I describe mineral based motor oils as a distilled, concentrated product. The impurities need to be removed from the raw petroleum. These oils are therefore less clean and contain many impurities. Again, the problem is really more of theory than practice but the difference does exist.

People repeatedly say that synthetic oils are more stable in a hot engine. I hear that they lubricate better. The answer is yes and no. Oil molecules do not break down, just the additives. Generally, the synthetic oils do not have VI improvers so have less to lose.

There are some properties of synthetic oils that actually result is less wear than with mineral oils. These help increase your gas mileage as well. Due to a reduction of internal friction of the synthetic oil your engine will run a bit cooler. Wear increases as temperature increases, all other things being constant.

A main advantage that the synthetic has over the mineral based oil is the ability to lubricate at startup. Both types of oil have the same specifications at 104 F, 212 F and 302 F. It is the startup viscosity characteristics that separate these oils. Synthetic oils do not thicken as much on cooling. They have better fluidity as the temperature drops.

A synthetic oil that is labeled as 10W-30 is less honey like as a mineral based 10W-30 motor oil at startup. They both have a thickness of 10 at normal operating temperatures. At 75 F the synthetic is not as thick. At 32 F the difference between the two is even greater. At 0 F the mineral oil is useless yet the synthetic works fairly well. Just keep the RPM to a minimum.

At temperatures below zero you will not be able to start your car with mineral oils while the synthetic oils may be used to -40 or - 50 F. Oils are so thick that the normal method of viscosity measurement is not possible. Instead we measure if the oil can even be pumped or poured. Again, we are only discussing a single category of oil, the multigrade 10W-30 API / SAE grade.

I took an except from the web about Mobil 1 oils. They compared a 5W-30 synthetic Mobil 1 oil to a mineral based 10W-30 and a 10W-40 in ice cold conditions. The engine turned over at 152 RPM with the synthetic 5W-30 Mobil 1. The 10W-30 and 10W-40 mineral oils turned over at 45 and 32 RPM respectively. Neither of those engines started.

Motor oil becomes permanently thicker with exposure to northerly winter type weather. This is more of a problem to mineral based oils. Waxes form. This is why it is a bad idea to even store a bottle of oil in a cold garage. It goes bad on the garage self just because it is exposed to the cold.

To recap, synthetic oils have similar characteristics as mineral oils at operating temperatures. The synthetic oil will however be less honey - like at startup even though it has the same API / SAE rating. Yet the synthetic 10W-30 weight oil is based on a heavier 30 weight oil while the mineral based 10W-30 oil is based on a thinner 10 weight oil. They are both similar at operating temperatures yet the 30 weight based synthetic is actually less thick at startup and much less honey - like at low temperatures. This is the opposite of what common sense dictates.

This is worth repeating: The synthetic 10W-30 weight oil is based on a heavier 30 weight oil while the mineral based 10W-30 oil is based on a thinner 10 weight oil. They are both similar at operating temperatures yet the 30 weight based synthetic is actually less thick at startup and much less honey - like at low temperatures. This is the opposite of what common sense dictates.

As one can see this is no easy topic. Are you with me?

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124 Posts
Discussion Starter · #4 ·
Molydisulfides are solids that settle out so cannot be of use as additives to engine oil but may be used in greases. However, some are selling these additives for oil claiming that new formulas keep the particles in suspension. If you mix sand in water and shake you get a suspension (by definition) but it still settles out.

The question is whether MoS2 can be dissolved in solution. Also, if it is any good for automotive engines applications. If it was as good as the additive manufacturers claim then it would be in every oil out there.

IT MIGHT BE. In recent years I have noticed that there is always a little bit of solid material in the bottom of a can of oil that has been on the shelf for a while. I always tell people to shake oil cans before putting the oil in your engine. Obviously you want all of whatever they put in that can to go into your engine.

I have never been able to get additive information out of the chemists from oil companies. They say it is proprietary. Yet if I went back to my old chemistry lab at the University of Florida and asked them to run some spectral samples I may be able to figure it out. So why do they refuse to tell me. I do not know.


124 Posts
Discussion Starter · #5 ·
Just as aside:
Early synthetic oils leaked often. Now they are not supposed to do that in any engine. Old seals do leak however, just because they do not last forever.

I would like to pull an except from my chapter 7. This is for those people who say they need to look for older oils that were "designed" for that particular, older engine. My feeling is that you should always use the most current, highest rated engine oil:

This is from ASTM D 4485-03:
.....The SH rating was used in oils starting 1993. The SJ rating started in 1997 while the SL became effective in 2001 oils. According to ASTM D 4485, SL rated oils are superior to previous oils and from:
X2.3.1 and 2: SL oil is for use in current and all earlier passenger cars, sport utility vehicles, vans, and light trucks. This SL rated oil can be used in engines requiring SJ and all earlier categories.

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