just wondering if anybody thinks 10w30 will flow just like 30 wt when hot. example: you have a car that has an oil pressure light that comes on when you are at a stoplight in gear when the oil is hot in the summer. switch to 30 wt and the light doesnt come on in the same situation. 10w30 in the motor and the light comes on. 30wt light is off. but they are supposed to flow at the same rate, right?

 

You are assuming that any oil that is Xw-30 and a straight weight oil are the exact same viscosity. That would be incorrect. The 30 part of the weight is more complex. For this answer I will use the kinematic ranges for clarification. An oil can be in the light or the heavy end of the range. For 30 weight oils that range is from 9.30 cST to 12.49 cST. So not all 30w oils, including multi-viscosity, are the exact same thickness at temp despite their being named a 30w. The high mileage oils, for example, use the high end of the range to provide a heavier/thicker oil which reduces oil consumption. Your experience indicates an engine that is worn. You might consider stepping up to a 15w-40 CH-4 rated oil like Delo 400 or Rotella T. It mosy likely will add a few miles to the life of your tired motor.

 

That's odd. According to James Fitch's book, How to Select A Motor Oil or Filter for Your Car or Truck (Noria.com), given a 10w30 and a straight 30 of the same 100C viscosity, the 10w30 should be thicker than the straight 30 at 120C. Of course this assumes no loss of viscosity from polymer sheer in the multigrade. What brand and type oil are you using? Does the 10w30 bottle include the words "Energy Conserving" in the API donut? Many of those are designed for temporary viscosity collapse in the bearings to reduce friction and obtain the "Energy Conserving" rating. Could try a non-Energy Conserving type, such as some of the so-called High Mileage oils. However, you may also want to get an oil pressure gauge to see what it's doing. They are only about $20 and fairly easy to install.

EDIT: I like Flash's answer much better. You apparently have a higher viscosity with the straight 30 than with the 10w30. (You could look them up on the oil company's website to get a ballpark figure for the viscosities.) What I have said above is also a consideration and the High Mileage oil would probably get you up to the viscosity of the straight 30, but you really should find out what the oil pressure is doing across the RPM range and then likely select an even thicker oil, such as a 10w40 or the 15w40 that Flash recommends, with consideration for winter low temp performance.

 

My take on this is maybe you are running at somewhat less than 100C. Most engines dont run that hot in normal operation. Straight grades are thicker at lower temperatures than multigrades. Look at the huge difference at 40C in the data sheets. An oil temperature gauge and a pressure guage would be necessary to study the relationship. Further complicating things is the fact that polymers and pour-point depressants make the viscosity index worthless due to non-linear nature of the resulting fluid.

Jim