quote:
Originally posted by JohnBrowning:
I posted this eariler in another thread but I thought it might help you so here it goes.
HTHS in law mans terms is the oil ability to resist shearing. SO the higher the number the harder it is to cut the oil into smaller dysfunctional pices. I am over simplyify here. An HTHS of 2.6 is the minimum for long term valvetrain wear and rod bearings. Some new lead matrix's are makeing this less of a problem for bearings but longterm valve train wear is still an issue. Your valvetrain wear and bearing wear in most applications drops significantly as you pass an HTHS of 2.9. It continues to fall of very sharply until you hit about 3.6 then it contines to drop off at a slower rate until you hit about 4. Once you pass an HTHS of 4 the only people that are going to see a reduceing in wear are those people severly pushing the limits of their engine. You really can not have too much HTHS as it is not an issue their is no down side to high HTHS numbers in terms of wear.
Now while HTHS is not linear to SAE viscosity it does track with viscosity for the most part. The big thing is that it tracks with the viscosity of the base stock. So if we have two identical base stock the one with more viscosity is going to have a higher HTHS. Different base stocks have different levels of shear resistance and the viscoity improvers also play a role. FOr example esters have better shear resistance then a PAO so an ester based 5W20 can have an HTHS of 3.3 while a PAO/AN based oil might be an SAE 0W30 and have an HTHS of 2.9. So we always have to compare apples to apples. If two oils of close to the same base stock make up have simalar HTHS numbers but vary different viscosity's generaly the oil with the lower SAE viscosity spread is going to be more shear resistant. A good example of this is GC 0W30 HTHS of 3.6 V.S. M1 0W40 HTHS of 3.6. Both of these oils due a good job in terms of generaly produceing low wear numbers. THe GC has never sheared while the M1 0W40 often shears and in some cases has exsaberated oil consuption issues.
The reason some people are leary of higher HTHS oils is that generaly HTHS does track with viscosity. So in order to get an HTHS above 4 you usualy have to go with a 10W40,15W40,15W50,20W50,25W70 oil in order to get above 4. You usualy give up more in HP, flow,pumpability then what you gain in wear reduction due to the law of deminishing returns. You might also consider it as a function of marginal return. Their is always going to be a sweet spot in almost any thing were you get the most bang for your buck. In general in most light duty passenger cars and trucks that sweet spot is between 3-4 with 3.6 seeming to be the most freq. and general sweet spot encountered! You can find oils from 5W20-10W40 that fall between 3.2-4 with no problem at all. So it truly is not an issue for someone to be able to find an oil that works best in their car. Most will tell you to use the lightest oil that turns in good UOA numbers for your car. This maximizes fuel saveings and will keep wear at a mangeable level.
Audi has been haveing oil related failures and is really specific about what they want. I would not use an oil in an Audi that did not havwe an HTHS of at least 3.5 personely. I belive Audi wants an HTHS of 3.6 min. but again ask some audi owners.
