I recently changed my oil, and I didn't have quite enough oil to top it off so I used HD-30 oil for my last .7 out of 4 quarts total. My car normally takes 5W-30 and after a little research I figured that it would be OK. Unfortunately, my car has a hard time starting up after sitting for a while now. I have to give it some gas to get the engine started up, which I'm concerned may be a sign that it wasn't a good idea to use this oil. On the other hand, I've read that it may just be the oil cleaning out the car, which I assume would be a good thing. It does spit out some black smoke from the exhaust when it has those difficult starts which I hope is residue being cleaned out. It is a '99 Saturn SC2 with 155,000 miles on it.
While I agree with Allan that you should clean out the oil and put what is recommended into the system, I think the problem is not the oil. I think you might have a leaking injector causing your hard start/black smoke on start-up issue. This is the reason I asked about the color of the smoke, as it makes a huge difference in the diagnosis. If you run some decent fuel additive through you tank (such as Seafoam, Royal Purple, or BG-44k), I think you'll see a marked improvement in how it starts. You may also try cleaning your MAF and throttle body and check to ensure the air filter is good. I'd need to know when the last time the O2 sensors and spark plugs were changed to tell you if you might need to replace those as well, but in most cars, the recommended change interval is around 100k miles.
HDF-30 as I understand it is for for engines like that found in tractors or off-road plant. Whilst not saying your starting and smoke is directly attributable to the addition of this oil (25%) I would think that it quite probable. Use an engine flush treatment whilst replacing this mix with its correct oil to put your engine on an operational safe side, even if only to eliminate any doubt.
Single grade oil is thicker than multigrade when starting a cold engine, more so as the temperature gets colder. In cold weather this might result in labored cranking and slow starts.
Once warmed up, a 30 weight is a 30 weight.
Engine oil cleaning should not result in noticeable changes in exhaust.