Rotary engines pump oil directly into the combustion chamber to lubricate the apex seals. Oil in the combustion chamber MUST burn or risk damage to the apex seals. Synthetic oils are (usually) designed to not burn inside of an engine and survive much higher heats the non-synthetic oils and will leave deposits on the inside of the rotor housings leading to damage of the apex seals.

Lower weight oils are usually not recommended for rotary engines because of the e-shaft and all of the bearings. Low weight oils at the temperatures the engine runs will be too then to provide adequate protection between the bearings and the e-shaft. As anyone can tell you, once the e-shaft goes the engine goes.

The additives that you mention that are used to "thicken" light weight oils gives you the same problem as the synthetic oils, it will not burn clean and will leave residues and deposits on the rotor housings.

Now, if you're running 2cycle oil in the gas (like I am) you CAN use synthetic oil since you no longer have to worry about oil needing to burn. Of course, I would still never use a light oil because of the e-shaft and bearings.

 

I understand how the oiling system works. I also know why there are synthetic oils that when burnt, leave a solid residue behind. Not every synthetic oil leaves a solid behind as a product of combustion. There are some where all the products are gaseous. If these additives are sulphates or phosphates like mobil one uses for antiscuffing agents and moisture control, then it will certainly leave a "deposit" when its burnt. What I want to know is what are these viscosity modifiers that icemark was talking about. I want to find out for myself what happens to them when they are burnt. I have heard this rumor before on this site, just tell where you aquired that information if you can, so that I can track down the source and find out if its based in fact.

 

Correct me if I'm wrong, but Icemark has posted that information before regarding conventional 10W40s leaving extra junk behind due to the specific polymers required to make that oil.

As for using a 5Wxx being bad, got anything to back that up? I know that the engine sounds smoother on startup with it. My thinking was that the engines I've had have sounded a tad rough on startup with 10W30 in cold->cool weather and 20W50 in cool->hot weather. Everyone knows the first 30 seconds of startup cause the most wear on typical engines because the oil is thick and perhaps not everywhere yet. Due to the higher year of a rotary a 5Wxx oil should thicken up quickly. Shouldn't a 5W50 be at 30 before a 20W50 is at 40? Also, synthetics are typically more effective at low temps.

 

As posted countless times:

as far as my sources for the lower base stock viscosity weights having issues (in all high reving motors, not just rotarys):

Toyota's TSB on 10W40 and 10W50 oils, and why they didn't outright say it, They suggested that customers using 10W40 and 10W50 oils in any Toyota/Lexus DOHC engine should have their warrenties questioned, and there will be sludge build up and high wear issues.

GM's complete outright suggestion to its dealers that the use of wrong weight oils such as 10W40 and 10W50 in any GM car (built after 1990) would void the warrenty for the same reasons as Toyota (sludge and deposit build up and wear).

And of course the VTR page on everything you wanted to know about oil: http://www.vtr.org/maintain/oil-overview.html

Remember with a conventional oil, a 10W40 or 10W50 uses a 10 Weight base stock, and long chain polymers that "unwind" to make the oil work like a 50 Weight when warm.

There are a considerably high amount of those polymers when using the low weight base stock over a middle weight base stock (such as a 20W50, which uses a 20W- considerably higher).

And shear strength alone is enough to stay away from a conventional 10W40 in any high reving motor. The parts tolerances typically do not tolerate any loss of film... something that happens when the long chain polymers rip apart.

Aside from even the shear issues... why would you want an oil with an increased polymer additive level than you need???

 

I still don't see any reason to not use a synthetic 5W50. If anything again reading the link you've posted, I'm more convinced than ever and the VI ratings confirm what I said above.

 

Wow. I just put 5w30 Penzoil in my car it runs fine, and the engine has 171k on it. should I move up to a higher weight oil?