A little correction here. 0-40 is not a 40wt oil pourable @ o deg. The lower number is the colder number, meaning before warm up, it's a 0wt oil, then as it warms up, it goes to 40wt.

Oil weight, or viscosity, refers to how thick or thin the oil is. The temperature requirements set for oil by the Society of Automotive Engineers (SAE) is 0 degrees F (low) and 210 degrees F (high).

Oils meeting the SAE's low temperature requirements have a "W" after the viscosity rating (example: 10W), and oils that meet the high ratings have no letter (example SAE 30). An oil is rated for viscosity by heating it to a specified temperature, and then allowing it to flow out of a specifically sized hole. Its viscosity rating is determined by the length of time it takes to flow out of the hole. If it flows quickly, it gets a low rating. If it flows slowly, it gets a high rating.

Engines need oil that is thin enough for cold starts, and thick enough when the engine is hot. Since oil gets thinner when heated, and thicker when cooled, most of us use what are called multi-grade, or multi-viscosity oils. These oils meet SAE specifications for the low temperature requirements of a light oil and the high temperature requirements of a heavy oil.

Now, comes the BIG debate.

Mobil 1, IS NO LONGER A TRUE SYNTHETIC! The laws were changed (when M1 sued Castrol) the definition of a synthetic oil was. So it's now a blend, here in the US. With mostly Grp III base stocks & no longer uses any PAO base. Amsoil is a pure synth, with almost all PAO, & GRP IV base stocks.

M1 is a good oil, but not a quality synth anymore. IMHO, I skip it, & get some Amsoil, Redline, German Castrol, or Shell Rotella T.

 

Thanks for the correction! I was working off of stuff I'd read about 25 years ago... I feel smart, again

 

Hmm I dont think castrol has 0W 40

 

they've got a synth 0w-30, tho. I used it last winter.