If it's a matter of oil getting by the valve stem seals, then moving to a
slightly thicker oil isn't going to do much, and here's why.
The oil is leaking down the valve stem after the vehicle is shut off and at operating temperatures. Thickness when the oil is cold will not affect the viscosity at operating temperatures, viscosity index improvers and long chain polymers are at work in a muti-grade oil.
A 5w20 is in the range of 6.9 to 9.3 centistokes at 100C.
A 5w30 is in the range of 9.3 to 12.5 centistokes at 100C.
A 5w40 is in the range of 12.5 to 16.3 centistokes at 100C.
A 5w50 is in the range of 16.3 to 21.9 centistokes at 100C.
A 15w40 is also in the range of 12.5 to 16.3 centistokes at 100C.
A straight 40 grade oil will also be in the range of 12.5 to 16.3 centistokes at 100C.
What is a centistoke>? It is a measure of one square millimeter of liquid per second. The standards of engine oils are created by the SAE and the test method is the ASTM D445.
Kinematic viscosity is the ratio of absolute viscosity to density.
The viscosity is very slightly different at operating temperatures, not enough to make an appreciable difference.
Reference this chart:
It lists the viscosity of different liquids referenced to water.
http://www.geotechenv.com/Reference_Pages/average_viscosities_liquids.pdf
Note that water at 65F is 1 cSt, mineral spirits is 2 cSt, and no.2 Diesel is 3 cSt.
Considering that most of you have dealt with these three liquids once upon a time, to the layperson, would it appear that diesel is three times thicker? Side note: beer is 1.8 cSt according to another chart.
If you were filling up a bucket with lots of little leaks, and switched from filling with water to diesel, do you think that it would take longer for the bucket to empty if it was filled with diesel instead of water? How much longer?
http://www.shambhaviimpex.com/redwood-viscometers.html
A viscosimeter uses a calibrated orifice ( think a carburetor jet ) and a heating element. Liquid is heated, and the time to pour through a calibrated orifice is timed.
WATER = 29 seconds. MINERAL SPIRITS = 31 seconds. No.2 DIESEL = 33 seconds, Redwood no.1 viscometer, all at 65F.
A thicker oil is only really much thicker at cold start up, the time where most engine wear occurs. A 5w - multi-grade provides good pumpability and will protect from cold starts.
Again, looking at the above, a 5w20 and a straight 40 grade aren't that much different when it is hot.
Now, if you have fuel dilution issues, a thicker oil can be a
very slight improvement over a thin oil to start.
If you have mechanical issues such as missing / cracked valve stem seals, going to a thicker oil is not going to do a damn thing for you. A thin oil will high detergent may free up stuck rings if the engine is in good operable condition, it gets up to operating temperature quickly, and you change oil regularly.
