I have seen a few oil posts on here recently and thought that I would share my expertise. I have posted this before but thought that it was worth reposting in case you missed it first time around.
The subject of synthetic oils in the rotary engine is a much debated topic on car forums and clubs around the world but when looking at the tecnical aspects of the argument, no one has ever convinced me (or at least not until now?) that they actually cause harm to the engine or shorten its life.
To start with some basics, we need to remind ourselves what an engine oil is designed to do. It must:
Oil must lubricate engine components so that they will easily pass by one another without a significant loss of power due to friction. At start-up, this is especially true. Once running, engine oil must create a film between moving parts to make them "slippery" which increases power, performance and efficiency.
Oil must keep engine components from coming in contact with each other and provides vital protection against wear. Engine oils also protect against corrosion of engine components.
If an engine doesn’t remain clean, it will not remain efficient. Deposits within an engine reduce fuel efficiency and rob your engine of performance.
Engine oil is responsible for a large percentage of the cooling that takes place within your engine. The radiator is only responsible for cooling the upper portion of your engine. The rest of the critical engine components are cooled mainly by the motor oil within your engine.
Contrary to popular belief and a healthy pinch of myth, all oils are not petroleum based or derived. Oil technology has moved on like everything in life as the chemists strive to create the perfect lubricants.
There are two main components that all engine oils are made of and they are basestocks and additives. The basestocks normally make up between 70 and 95%. Additive chemicals are then added to enhance the positive qualities of the basestocks and to overcome any negative qualities present.
There are two main types of basestocks used, petroleum and synthetic. Petroleum basestocks are a purified form of crude oil and have been used as automotive lubricants since motor oils were first developed. Synthetic basestocks, on the other hand, are chemically engineered in a laboratory specifically for the purpose of lubrication. They are engineered from pure compounds that contain no contaminants which must be removed via purification. Synthetic basestocks have been around for many years but have only been used in the automotive market for the last 30 years.
The main types of oil basestocks commercially available in the market today apart from the traditional mineral oils are as follows:
Hydrocracked (also referred to as Molecularly Converted Mineral Oils)
These are so pure and refined, they can now be called synthetics. They are not made from true synthetic basestocks (as previously mentioned) but have so little in common with traditional petroleum basestocks, that it doesn’t make sense classify them as petroleum oils. Petroleum basestocks can be put through a super-extreme refining process called “hydrocracking”. It is completely legal for lubricants manufacturers to label these oils as "synthetic". These are extremely high performance petroleum basestocks, will not perform to the same level as true synthetic basestocks like PAO (poly alfa olefins) or Esters.
Hydrocracking results in a basestock which has far better thermal and oxidative stability as well as a better ability to maintain proper viscosity through a wide temperature range - when compared to Mineral oils.
The most common types of Synthetic basestocks.
Poly alpha olefins (PAO\'s)
These are the most common synthetic basestocks. Many synthetics on the market use PAO basestocks exclusively. Also known as synthesized hydrocarbons, they contain absolutely no wax, metals, sulfur or phosphorous. They have extremely low pour points and are very thermally stable. One drawback to using PAO\'s is that they are not as oxidatively stable as other synthetics unless properly additised.
These synthetic basestocks offer many of the same benefits of PAO\'s but are more varied in structure. Their performance characteristics vary more than PAO\'s. When chosen carefully, diesters provide better pour points and thermal stability than PAO\'s and are a little more oxidatively stable. Diesters also have very good inherent solvency characteristics which means that not only do they burn cleanly, they also clean out deposits left behind by other lubricants, without the aid of detergency additives.
They are also surface-active (electrostatically attracted to metal surfaces),
These synthetic basestocks are similar to diesters, but slightly more complex. They have a greater range of pour points than diesters, but some polyolester basestocks will outperform diesters. They are also surface-active.
Premium quality synthetics are normally a blend of more than one type of PAO and/or PAO basestocks with diester and/or polyolester in order to create a basestock which combines all of the benefits of these basestocks.
The Synthetic Myth.
There are many oils on the shelves today labeled as “synthetic” but in the true sense of the word they are not. They are the “hydrocracked” petroleum oils previously mentioned and although generally superior to mineral oils they are not of the same quality as true “made in the laboratory” synthetics.
So, why are these highly refined mineral oils called “synthetic”? It was all decided in a legal battle that took place in the USA about ten years ago. Sound reasons (including evidence from a Nobel Prize winning chemist) were disregarded and the final ruling was that certain mineral bases that had undergone extra chemical treatments could be called “synthetic”.
Needless to say, the marketing executives could not believe their luck! They realised that this meant, and still does, that the critical buzz-word “synthetic” could be printed on a can of cheap oil provided that the contents included a few percent of “hydrocracked” mineral oil, at a cost of quite literally a few pence.
Synthetics and the rotary engine.
How many times have we heard it “do not use synthetic oil in the rotary engine?”
Too many and there will be many more but, it’s a pretty meaningless statement unless we as rotory engined car owners and the person making the statement fully understand the meaning of the word “synthetic”.
As I have stated before, most oils on the shelves today do not contain true synthetics, they are merely highly refined mineral oils and as such are just a high performance mineral oil.
I have done much research into this topic since I picked up my shiny Velocity Red RX8 nearly 12 months ago and I have to say with no definitive conclusion. I know of RX7 and RX8 Owners that use fully synthetic oils in their rotories with no evidence of any ill effects to the engine, it’s not surprising as they are without doubt superior lubricants.
However, when I questioned Mazda UK about the use of Fully Synthetic oils we received the following reply.
“The Mazda RX-7 engine contained rubber parts that would be affected by synthetic oil so dealers were told not to use it in rotary engines. We asked on Mazda RX-8 training if this was still the case for the new Renesis engine and were told that although the rubber parts in the engine were all now believed to be synthetic oil compatible Mazda Corporation had not done any testing or evaluation with synthetic oil.
Mazda Corporation has developed and tested this engine using mineral oil and has recommended its continued use in service, I would consider that straying from a proven and tested specification is not some thing that we could endorse. Although the oil may be a superior lubricant, there is no requirement for it in an engine with only three major moving parts none of which are reciprocating and all of which are in near perfect balance.”
In itself this answer is not really conclusive, the seal compatability problem seems to have been solved but it’s not been tested. So I decided to take a trip all the way to Stoke (Belper to be precise) to talk with the Silkolene technical boffins and the chief chemist about Mazda\'s reply and synthetic compatability.
Their first comment on Mazda\'s statement was that depending on what synthetic they are referring to i.e. PAO (Poly Alpha Olefin), Polyolester or Diesester the seals would be affected in different ways.
PAO\'s always shrink seals
Esters always swell seals
They (Silkolene) tested a mixture of PAO/Ester in the early Norton Rotary Bikes as they were asked to formulate a "proper" synthetic for Norton and they found that the correct mix (not 50/50 I might add) had the affect of no swell or shrinkage of seals. Oils have come a long way since.
They told me that "proper" synthetics PAO/Ester blend types are of great benefit to the Rotary engine and the "shear stability" of these oils added to the "polar" benefits of ester make these oils perfect for the engine or at least this was their opinion.
Their reply to Mazdas comments on the engine itself were as follows:
This does not make sense, Esters have been used in the aviation industry for more then 50 years..................Think about it!!
Anyway, my conclusion is that "true" synthetics are better and that\'s a fact but they cost more and are not so readily available so in a car that uses around 1 litre per thousand miles was this a commercial decision as the oil\'s not in the engine long enough to "shear" or "degrade" to any great extent?
So the next question is what do I use?
I follow the handbook and use a "hydrocracked" (petroleum based highly refined mineral oil) which is in my opinion and from a chemists point of view labelled as a "synthetic" but it\'s not as the basestock is petroluem. It is a 5w-30, a 10w-30 can also be used for different ambient temperatures.
As for the RX7\'s well it\'s personal choice really and as I\'ve said before, synthetics are better but most are not true synthetics so as long as you stay clear of 100% Poly Alpha Olefin based oils (Like Mobil 1 as an example) which would give the highest levels of seal shrinkage you will be fine. An oil with a good percentage of ester basestocks will counteract this shrinkage.
There is no doubt that "hydrocracked" oils that are either Fully or semi-synthetic are fine to use because they are merely highly refined mineral oils will give you better protection than a plain old mineral oil. Silkolene actually recommend the use of their 10w-40 semi synthetic products in the RX7.
So, there it is, complicated until you know what "synthetic" really means. Hopefully this sheds some light and some food for thought on this slippery topic.
If anyone has any questions, please feel free to ask.