From a viscosity standpoint, an SAE 10W-30 oil, whether regular petroleum oil or synthetic oil, is still a 10W-30 oil. Viscosity is the most important characteristic of oil. Viscosity refers to the thickness of a liquid, and is measured for automotive purposes according to the Society of Automotive Engineers test J300.
The "10W" aspect of SAE viscosity refers to the fluidity of the oil at -20deg. C measured in Centipoise (cP). The second number, the "30", refers to the fluidity of the oil at 100deg. C. (normal operating temperature of an engine) and is measured in Centistokes (cSt). The first number (in this case the "10W") would be indicative of an oil tested at a cold cranking temperature of -20 deg. C with a maximum 3500 cP. A "5W" oil would be measured at -25 deg. C with a maximum of 3500 cP.
These test are relevant in testing petroleum oil in that they are indicative of the pourability of oil at lower temperatures. As an example, a 10W oil has a maximum allowable pour point at -32 deg. C. In other words, the pour point is the temperature where the oil will no longer freely flow.
With a synthetic oil such as AMSOIL, the viscosity measurements are the same. An SAE 10W-30 petroleum oil has the same viscosity at 100 deg. C as a synthetic oil at 100 deg. C. Where the difference comes in is what happens to the viscosity as the oil gets below the -20 deg. C range used in the SAE test in determining actual pour point. AMSOIL SAE 10W-30 oil has a pour point of -48 deg. C (-54 deg F). The fluidity is maintained at the lower operating temperatures and will be pumped through the engine to reach critical components faster.
In conditions such as you describe, however, I would recommend you consider ASMOIL's Signature Series 0W-30 oil. At operating temperature, it acts just like any other SAE 30 grade oil, but the pour point of this oil is -60 deg. C (-76 deg. F). You will see a significant improvement in cold cranking and much less stress on your starter and battery.
Another AMSOIL product that you should consider is our 100% Synthetic Automatic Transmission Fluid and/or gear lube (depending on the requirements of your transmission). Although you are conscience of putting a heater tape on your engine, few people consider the transmission. If you have ever experienced the sluggishness of your transmission in very cold weather, you will recognize that the ATF fluid also gets very thick in cold temperatures, and the resistance to fluid movement causes much wear on transmissions on cold starts. AMSOIL's Synthetic ATF has a pour point of -60 deg. C (-76 deg. F) and also maintains its stability much better than petroleum atf in high temperature ranges. Petroleum atf usually starts to degrade at 170 deg. F, and only maintains borderline performance until 270 deg. F. AMSOIL's Synthetic ATF maintains stability to 270 deg. F and beyond. You will see a noticeable performance increase at both low and high temperatures.
In differentials, gear lube likewise gets very thick at cold temperatures. The unique designs of differentials uses the ring gear action to move the fluid into the bearing areas at the pinion. I have seen a video of a test that utilized a truck differential which showed that at -15 deg. F, it took almost 20 minutes for the gear lube to become fluid enough to reach the pinion bearing. With AMSOIL 100% synthetic gear lube, the fluid was at the bearing in well under 2 minutes. Our 75W-90 gear lube has a pour point of -40 deg C.
You may want to check with the manufacturer or supplier of the oil you are now using to find out what the pour point of their oil is. As it is an SAE standard, it should be available.
