Originally Posted By: lawnguy
What does the 10 in 10w 30 mean?
The oil has a viscosity of a 10wt oil at 40C, and it behaves as a 30wt oil at 100C.
In the world of modern synthetics, there is really no such thing as a straight 30wt. The oil's viscosity index meets the 10w-30 requirements without any additional index improvers. The old conventional oils were made thin (like a 10wt), and viscosity/index improvers were added to thicken it up when it got hot to behave like a higher viscosity oil. The only difference between a straight 30, and a 10W30 is the viscosity when cold.
There is no advantage to running a straight 30 vs a 10w-30 when you are using a synthetic. It is for all purposes a 30w oil and will behave as such when hot.