Lister D smoke

Hi , i have noticed that on 2 of the 3 i have had the longest do smoke a little when they get to full temp , Which makes me think that as the 2 are the longest owned and hence had more eager bouts with the oil can . I took the rocker cover's off today the newest was greased very well as were my older 2 or longer owned 2 , but around the valve heads  i must admit were overly covered in oil , Would i be correct in thinking this could cause a little smoke to be puffed out , its not clouds , its just there a little , I intend to dry them out and a general adjust all round am i ok not to panic and right to believe its that or is it the start of something a little more sinister   of is it