If it's a hot oil smell after you've been giving it stick then it's nothing too much to worry about. Older cars all tend to do it. A lot of rocker covers tend to weep with age and it all heads south onto the exhaust headers which is where the smell usually comes from. A lot of dirt and grime on the engine causes a lot of the hot burnt smell too after you've given it a roasting.
Any other suspicious smell might be worth intvestigating for piece of mind.