Nothing wrong with single weight HD oils. Not one single thing. As long as you run one with the zinc content or run a zinc additive, you'll be fine. That's the type of oils they ran when they were new and they lasted hundreds of thousands of miles. Modern oils are even better. Multi viscosity oils can only be one weight at the time anyway. A good 30 weight for warm climates and a good 40 weight for cold climates will do just fine. The most important thing is to keep the crankcase full with SOME kinda oil. I would advise against the cat, BTW. Cats are designed for engines running MUCH leaner than these older models.The extra unburned fuel in the exhaust can cause the cat to literally melt itself and burn itself up and possibly cause something close by to catch on fire as well.
