Whatever oil you choose to run has to form a film or cushion of oil to keep the the bearing off the journals. The pressure keeps the oil with that film to do so. If the pressure becomes too low it can't do so and the bearing drags on the journal causing wear. The clearances of the bearings is what determines the viscosity to supply need to form this cushion.
The viscosity of the oil under severe heat is needed to keep this cushion up, so if 15-40 or a 20-50 is used the first number is what is performing the job, the second number is what it does under severe heat and usage to maintain the the job the first number of the oils weight.

Multigrades
Multigrade oils typically begin as base oils, such as 10W. Then viscosity-index modifiers (polymers) are added in an effort to stabilize the viscosity. This allows a 10W40 oil to flow like a 10W at cold temperatures and a 40W at higher temperatures. In other words, multigrade oils are formulated to pass viscosity tests across a range of weights. For example, 10W30 meets the requirements for 10-weight at cold temperatures and 30-weight at high temps.
The multigrade oils' viscosity modifiers are long-chain molecules that lessen the change of viscosity with temperature variance. In the past, the polymer additives (used to thicken the oil) were sometimes susceptible to viscosity loss. Permanent viscosity loss occurred when high shear forces (such as the relationship between the main bearings and the crankshaft) actually break the polymer molecules into less-effective smaller pieces. On a similar note, temporary viscosity loss also occurred when the polymer molecules aligned themselves in order to create a path of least resistance.
Fortunately, today's additive packages have improved oil's shear-resistance. However, oils with the same rating from different manufacturers can exhibit different viscosity ratings in an operating engine, depending on the shear stability of their viscosity-modifying additives.
For technoids, weights are defined thusly (stokes and centistrokes are measurements of viscosity):
"SAE 30 is SAE 30 no matter what the "W" prefix number is: 0W, 5W or 10W. This viscosity in centistokes (cSt) @ 100 degrees C is with the minimum of 9.3 cSt and a maximum of 12.5 cSt.
"SAE 40 is SAE 40 no matter what the "W" prefix number is: 5W, 10W, 15W or 20W. The viscosity @ 100 degrees C is within the minim of 12.5 cSt and a maximum of 16.3 cSt.
"SAE 50 is SAE 50 no matter what the "W" prefix number is: 5W, 10W, 15W or 25W. The viscosity @ 100 degrees C is within the minimum of 16.3 cSt and a maximum of 21.9 cSt.
"SAE 60 is SAE 60 no matter what the "W" prefix number is: 10W, 15W or 25W. The viscosity @ 100 degrees C is within the minimum of 21.9 cSt and a maximum of 26.1 cSt.
"There is no SAE 70 and no one is likely to make one with a "W" prefix number although it is possible using a synthetic base oil. This viscosity is identified as Grade 70. The viscosity @ 100 degrees C has a minimum of 26.1 cSt and no maximum."
The difference between a multigrade and a singlegrade oil: The singlegrade can't pass the low temperature viscosity test. If it did meet one of the following "W" viscosities, it would be a multigrade.
Singlegrade oils will become obsolete for performance engines in the future. We dropped SAE 30 and SAE 40 because SAE 10W40 does everything 30 or 40 can do—and some things the straight grades can't do—like increasing horsepower. If an off-roader doesn't like 10W40, then use 20W50. It can do everything a 10W40 can do except pass the sub-zero viscosity test at -20 degrees C.
Multigrade viscosities are run at six different sub-zero temperatures. When a racing-oil designer puts a formula together, he has to know the viscosity at 100 degrees C of every component in the additive composition. He has to have a target viscosity objective for the finished oil in each SAE grade. Once a formula is established, the technician who supervises the blending has to duplicate this formula in the correct proportions every time the product is blended. The viscosity at 100 degrees C has a plus or minus written into the oil's quality-control specification.

Salesmen, I asked what the "W " meant  on the label. He had no answer he didn't know and this was with a reputable oil distributor.
So what I am reading here are direct quotes from written passages, which is good. The more knowledge the better understanding of oils.
Growing up we were in the transportation business and the knowledge of oils was terrible. We used about everything coming down the pike. In those days they all wore ten gallon hats and criticized every body's product. We learned the hard way we were getting engine changes instead of oil changes. We thought the more expensive was better, not true, the oil not being no detergent was in fact 13% detergent and was ruining our engines. Remember 60 years ago things were different and paraffin oils was getting scarce and asphalt base oil came into the market now synthetic, quite a change. So too really understand what you are using is important. Remembering the most expensive is not necessarily the best for your application. There is a world of difference in engines, two stroke, four stroke,gas powered, diesel powered, propane powered that use all difference base oils. I agree light weight oil flows better returns faster and needs less power distribute the flow, so now the properties in the oil are very important especially in sever conditions.

Shell Rotello

If ya wanna go really fast you should use this kind...

Im not a 100% on the roller but still delo is for a diesel...sounds like to me you are bandaiding an issue with a rear main that should be addressed....im not saying your setting yourself up, but i tend to use things on what they are intended for

Also a lot depends on the engine compression the higher the comp ex. 13,14, to 1 the top bearing get squeezed tighter rather than a 8.1 comp. the wear when running a high comp engine the top bearing bears the brunt of wear, like a diesel.
The more the engine is put together the more better oil is necessary.