5-30 in any stovebolt is a HUGE mistake. These engines aren't designed for water-thin oil. I believe any oil with a lower viscosity number below 15 will tend to hammer out bearings and make the engine sound like there are dozens of little guys with hammers in there trying to break out.
I use 15W-40 in everything I own, from lawn mowers to farm tractors. No oil-related failures yet!
Jerry
From the factory trucks came with SAE 10W.. that is 'water thin' (COLD the viscosity is 10.. not hot.)
A 15W40oil really isn't giving you any 'cushioning'.. its only increasing the oil pressure (at the gauge) and making it harder for the bearings to create a boundary layer film (viscosity = RESISTANCE to flow).
In a perfect world we would have the thinnest possible oil that would not shear or boil away under heat... generally that's a 20 weight oil... maybe a 30 weight in HOT climates.
A good 5W20 or 5W30 oil is probably the best choice. Truth be told any oil is better then no oil.
