Oil viscosity ranges and time

[rjacket]

I have read a few times on this site that xw30 oils shear down to xw20 over time, and that xw20 is more stable. I also looked at the ranges for 100c temperature viscosity and found the following

xw20 5.6-9.3
xw30 9.3-12.5

I also read that mineral oils are "naturally" an x weight oil ie 5 or 10. Viscosity improvers are added to achieve the 20 or 30 or 40 at 100c and that over time these viscosity improvers are used up so the oil becomes progressively thinner at 100c.

Then I read that synthetic oils are "naturally" the 20 or 30 or 40 weight and something else is added to them so that they stay thinner at cooler temperatures.

Then I read that oil naturally thickens over time, especially mineral oils exposed to cold.

So please help me understand the following

1) Do only mineral oils thin over time due to loss of viscosity improvers or do synthetics do this as well. Ie is it true that if you buy a synthetic xw20 or xw30 that it is more likely to remain a 20 or 30 than the mineral oil because it has far less VI's in it.

2) Is it true that oils will thicken over time also? Is this true for both mineral and synthetic? If it is true, then is the rate of thickening faster or slower than the rate of thinning due to loss of viscosity improvers? Is it possible that a synthetic actually has a greater tendency to move away from it's initial viscosity due to it thickening but not having anything making it thin compared to a mineral oil which is thinning from loss of VI and thickening due to natural effects

3) The range for a xw20 is quite large. If Ford & Honda have spec'd 5w20, then does that mean it is safe to run something with a viscosity of 5.6 in it? Given that many of the engines are unchanged from those that were once spec'd as 5w30, then the possible range that the engine can run at is huge ie 5.6 to 12.5.

4) If oils do shear down, then for an engine that is spec'd as 5w20, would the best 100c viscosity to aim for be something in the middle of that range ie greater than 9 which means a heavy 5w20 or a light 5w30. In that case you would be certain that your viscosity would be always in the middle of the range for that engine design from fill to drain.

 

[Bruce T]

I can't answer all these questions, but I don't want you to sit at the bottom of Page 4 without a single reply. This has been discussed before, but I don't know how to find the old threads.

Oil does get thinner from the shearing of viscosity index improvers, but also from the effects of fuel dilution. Some oils shear faster or slower than others. When a oil gets completely used up, it will start to thicken (as in sludge). If I'm wrong, I'm sure others will correct me.

 

[rjacket]

Originally Posted By: Bruce T
I can't answer all these questions, but I don't want you to sit at the bottom of Page 4 without a single reply.

 

[modularv8]

I will take a stab at your questions.

1) Both shear over time. Synthetics hold up better. The viscosity improvers is one factor leading to "thinning" but is the main reason conventionals lose viscosity quicker than synthetics.

2) Both will thicken (oxidize) over time. However, synthetic is more resistant to oxidation. Leaving an oil too long in an engine accumulates contaminants while volatizing, leading to thickening (worse case scenario, sludge).

3) Yes. The viscosity range for a given weight can be large, but the HTHS has a minimum for each weight. Therefore a certified oil (GF-4) viscosity of 5.6 cSt must still meet the minimum HTHS of 2.6 cSt for the 20 wt. grade. Some engines you can run a 30 wt., others you can not.

 

[Mustang Man]

Also, using 5w30 as an example, in dino it is a 5w oil with VI improvers so it is 30 at operating temperature. In synthetic, it is a 30 weight oil. It is how the synthetic is created in the lab that makes it flow better at lower temperatures.