I don’t quite agree with you. I don’t think it is rod knock, that would be loud and constant. If it were a collapsed lifter, the heavy oil would aggravate it. It sounds most like a loose and worn out timing chain. I don’t think heavy oil will make it much quieter or help it last any longer.
An oil change would be a good idea though. Since its close to summer, straight 30 HD might be warranted but a diesel rated 15w40 would be my choice for this case. It has the best lubricity. Diesel oil has a couple of components no longer allowed in gas engines due to alleged damage to catalytic converters.