Originally Posted By: fpracha
Originally Posted By: Gokhan
I understand that there is a concern that since these rotary engines normally burn oil in order to function and the synthetic oil doesn't burn as well as dino oil (both of which are true), it's recommended to use dino in this engine.
Perhaps any 10W-30 dino should do well then. You could also use 10W-40 dino but it will leave more viscosity-index-improver-polymer deposits behind, although the consumption will be less; so, maybe it's overall a better thing. I would say if the consumption is not excessive with 10W-30, use 10W-30.
15W-40 HDEOs are a different story -- they leave behind different kinds of antiwear layers -- not necessarily a bad thing but not necessarily a good thing either, especially if you change back and forth between HDEO and PCMO. Also, 15W-40 is rather thick and you may see some sluggish performance and relatively poor fuel economy.
10W-30 dino oils of today almost don't shear at all and they would probably be equivalent of 10W-40 oils of 1980s after they sheared.
So back to the 15W-40 again... do these oils leave VII deposits or much more of other types of deposits?
In your experience are these deposits mainly generated during much extended drain intervals or even during shorter drain intervals ?
And will these 15W-40 oils cause the sluggishness and poor fuel economy due to original viscosity being thicker, OR due to the deposits they leave in engines?
Thanks!
Sluggishness and loss of fuel economy is due to the higher HTHS viscosity, which is what mostly determines the oil-caused engine friction. 15W-40 has an HTHS viscosity typically around 4.3 cP, whereas 10W-30 has an HTHS viscosity typically around 3.1 cP.
I was mainly talking about the antiwear layers, as HDEOs tend to use somewhat different formulations of ZDDP than PCMO, and these layers will be somewhat different. As far as engine deposits are concerned, I never had a big problem with them when I did use 15W-40 for a long period (six years or so). I think with thicker oil, it's a little bit harder for the oil to flow and therefore it's a little bit easier for the deposits to form. But then thinner oil has a higher NOACK volatility, which may increase the tendency to leave behind some oil-related deposits.
VII deposits depend on the type of oil used. 10W-40 has more VII than 10W-40 and it also has more VIIs than 15W-40. Therefore, as for as VII deposits are concerns, which could lead to sludge, I would most worry about 10W-40. In fact, I believe GM explicitly recommended against 10W-40 in the early days precisely for this reason. But then dino 5W-30 also has a lot of VIIs, and it could also be more sludge-prone.
With synthetic oil, deposits could only be a worry in turbocharged engines.
Coming back to my experience, switching from 15W-40 HDEO to first 5W-20 dino and then 0W-20 synthetic, I am seeing the best performance with the 0W-20 synthetic. I think the thinner synthetic has really cleaned the engine and it's also giving better performance due to lower HTHS viscosity. The engine is certainly running smooth, but in most part it's thanks to good maintenance, especially of the carburetor and emissions. Note that I wouldn't be able to make the switch to a thinner oil if I hadn't replaced the valve-stem oil seals because the oil consumption would be too high. Before then I was adding about 1 quart every 1,000 miles and with 0W-20 I would have to add about a quart every 250 miles. Now, the oil consumption is almost zero.
The next step will be a UOA so that I could see if there is any unexpected wear due to use of thinner oil.