[beckerb]

The original reason for using a low viscosity spread oil:
With dino based oils, as others have mentioned, you start with a lighter weight base oil. To achieve a multigrade oil, you need to add VII's (Viscosity Index Improvers). These VII's are long-chained polymers. When cold they bunch up, when hot they extend, but only to a limit, and by doing so, they limit how much the viscosity decreases.

The reason to use a low viscosity spread for DINO oil, is because the lower the viscosity spread, the lower the amount of VII's needed. The reason folks used to recommend things like 15w40 is because some companies could make dino 15w40 oils that used little to no VII's.

Why are VII's bad? Simple. They work because they are molecular long chained polymers that depend on their restricted uncoiling to work. Unfortunately, these VII's can chemically break down AND the molecules can literally be sheared mechanically/physically. Once physical/chemical shear has happened, the oil will have a permanently lower hot viscosity (which tends to mean less protection).

This is also one of the reasons the old fogies (like me) would use 20w50 oils; one was for for the lower viscosity spread. BTW, I'm skeptical of anyone who tells you something like "keep the spread to 25 or lower". It's not the difference from subtracting the numbers. It's the difference from dividing them. When folks don't understand this and STILL want to give advice, you need to question their understanding of tribology. A 20w50 has a 2.5X spread. Any VII's needed only need to account for a 2.5X difference. A 5w30 on the other hand only has a 25 weight numerical spread compared to the 20w50's 30 numerical spread, BUT the 5w30 needs VII's to maintain a 6X spread. You can do the math for other weights.

Suffice it to say, some synthetics are capable of 10w40 without VII's (Red Line is one), and NO synthetic can currently be formulated for 0w40 without a decent amount of (too much IMO) VII's. Mobil1 0w40 sheared like crazy in just 1500 miles in my car and that didn't include any track time.

The other reason for old schoolers running 20w50; the base viscosity is higher, so when (dino 20w50's usually still have some VII's) the oil shears, you still have a higher base weight (meaning at the absolute worst, a dino 20w50 won't shear below a 20 weight).