How to choose oil viscosity

[bobcowan]

I use 5W-30.

5 = the viscosity when it's cold. I forget exactly what temp that is, but I think it's 0*C. If you drive in cold weather (,50*), don't go higher than 10. You need to get the cold oil up into the cold engine as quickly as possible.

Cold thick oil can also damage the oil cooler if you blip the throttle.

W = "winter", not weight.

30 = it will have the viscosity of 30 that has been heated to 100*C. That doesn't mean it has a viscosity of 30 when hot.

You need enough viscosity to give the proper oil pressure, whatever you decide it should be. Thicker oil wastes energy, adds wear to drive parts, and provides no additional benifit.

Keep in mind that the oil pump has a bypass spring. If you're getting 50psi with 5W-30, you won't get much more than that with 20W-50.

[mtpockets]

The viscosity will determine how easily the oil is pumped to the working components, how easily it will pass through the filter, and how quickly it will drain back to the engine. The lower the viscosity the easier all this will happen. That is why cold starts are so critical to an engine because the oil is cold, and so relatively thick.

The lower the viscosity, the less the load the oil can support at the bearing on the crankshaft. The higher the viscosity, the better the load it can support. There is a trade-off, since the higher the viscosity, the more the drag at the bearing, and hence, potential power loss, or increased fuel consumption. So a compromise is chosen to minimize power loss, but maximize load support.

Until the 1960s it was quite common to change the oil depending on the season with oils as low as SAE 5W for very cold ambient conditions and SAE 30 and 40 grades would be typical for Summer use, and sometimes a SAE 50 grade would be substituted if the engine started to use more oil. Multi-grade oils were developed to allow year round use as a result of artificially raising the VI (viscosity index) of the oil.

Scientists discovered the use of polymers and the fact that these would expand with heat. VI Improvers are polymer material that expands as the oil warms up. This does NOT thicken the oil as is often stated. It merely slows down the rate at which oil thins out as the temperature rises. The base oil in use in a multi-grade (aka multi-seasonal) is the first number (such as 10W or 20W), whereas the second grade is the viscosity achieved using the VI Improver (such as 40 or 50). 20W50 is a base oil of SAE 20W and the viscosity grade at running temperature is SAE 50.

[Ant]

My engine builder recommends a 15-40 wt oil for break in and uses oil like Schaeffers 705 for racing.

I have spoken to oil chemists and they state a 20-50 oil running at the correct temperature such as the Schaeffers or Penrite gives a better protective film for racing or at least a bit of built in insurance. My car has an oil tank heater so I believe that takes care of any cold start issues, also Schaeffers 705 racing oil has excellent cold start features. We in NZ have generally a colder climate than Aussie or California etc so I wouldnt be against using a 40wt oil. My engine has been dynoed on Chevron Delo 400 15-40, I spoke to Chevron and this oil is specc'd for gasoline engines as well as diesel so in effect Joe Bloggs on the street may say its a diesel oil in actual fact he doesnt know what he is talking about!

I am intending on using Schaeffers 705 as being a race oil it is also suitable for extended oil changes which I wouldnt do, plus it has the good zinc level.

Here in NZ Grant Brennan has tested different oils in a RACE ENGINE and has come up with the results below relating to the enclosed weblink.
Schaeffers 705 Racing Oil 20W50 is the highest quality semi-synthetic multigrade oil for high performance racing engines available. It will give a competitive edge through increased horsepower and engine protection. It is specially formulated to reduce friction and wear, increase engine efficiency.
"We have tested Schaeffers against the competition and the results are conclusive. We wouldnt run it in our race engines if we didnt believe it was the BEST product available". GRANT BRENNAN - Brennan Racing Ltd

http://www.bernoil.com/resources/Bre...estResults.pdf

My only experience is with Valvoline SAE 50 and all that did was cost me oil pressure, I changed to Valvoline synracing 15-40 and that still dropped oil pressure down to 35-40psi. I was recommended Castrol RS 10-60 and found this oil to be extremely temperature stable etc in my F5000 engine started off 70psi cold and NEVER went below 60psi on the hottest day, nowadays I feel the 60wt is a bit heavy and make cost a little power, the question is would you actually notice it?????

Also with my Ford Laser 1.3 it has done 490000K nothing other than 10K oil filter changes, belts and plugs on mineral 15/40, car isnt garaged with frozen over winter starts. That tells me the oil is getting to where it should be on cold startup and the only difference 5-30w may do is make it slightly more economical. Some forms of racing use light weight oils probably due to the rpms and it wont affect the short life the engines have. I have to go with oil manufacturers recommendation for larger older tech engines and they generally recommend at least a 40wt hot oil.