Howdy John 15-40 should be ok to use. The oil will be at 15 on cold winter mornings and at 40 in a running engine. I use the same oil in my deisels and if I come up short I use it in anything that needs some added.
Around here straight 40 weight would be ok all year. Specially in a worn older engine where the clearences are opened from wear.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

professional bovine relocation specialist