As a kid all chains were lubed by using an old paintbrush and a can of drainoil.
I still use the same method for the chain baler.
For other things I use a spray on chain lube.
Kubota M7040, Kubota MX5100, Deere 790 TLB, Farmall Super C
I can't imagine why a manufacturer would use 15W40 on a chain when the stickier "chain lube" works so much better. Maybe 15W40 flows better in an auto oiler? On my Brillion seeder chains I use chain lube and it stays put for hours.