Actually, the 10w does not mean the oil flows like a 10 weight when cold. Compare the cold flow rates of 10w-30 and 10w-40, the 10w-30 will flow freer then the 10w-40 at the same cold temperature. the 10w is not the same between the two.

That said, I've run 20w-50 off and on for years, coming from the old engine world of sloppy tolerances. If the engine will hold adequate pressure with a thinner oil, you're better off with it. Thinner oils flow better, getting to places that need it faster, especially when cold. It takes less power to pump a thinner oil, and it's less likely to be sheared. It's been a hard habit to break though.

I tend to run a 10w-40 or 15w-40 oil in the summer on many engines now (as opposed to 20w-50), with a 10w-30 or 10w-40 for winter. Can't say that I detect any appreciable change in gas mileage or oil loss/consumption in the summer times when compared to thicker oils. But in the winter, the engines are much easier to start, and clatter less for the first minute or so.

Bulk oil is a misnomer, and be carefull of that. Castrol GTX 10w-40 is identical, be it in a quart can, 55 gallon drum, or multi-thousand gallon train car. It's bulk storage doesn't change the oil itself. Now that "no name" special that comes in the drum for $20 is a different story. It may be a good oil, but you don't know, so you're gambling if you use it.