If you've hidden the thread then you won't see replies, but I'm going to put my comment in anyway 
Obviously, this is going to be something that is influenced by your relationship with step grandfather ~ no-one expects you to call someone who has perhaps come along when you are an adult, 'Dad', but, as some others have said, the dc have the same relationship with this 'older person that they see quite regularly' and they just need a name to call them.
I would suggest that is why the OP has started this thread.
In some cultures (I'm not going to presume it's all African Cultures, or all Asian Cultures, but certainly in family friends from Cameroon, Zimbabwe, and India they all do this) all 'older' people are called a name sorry, I can't remember the words
that is equivalent to 'Grandparent', it is an honourary title given as a mark of respect and live, the way we used 'Aunty' when I was growing up, but is ridiculed on MN these days. The person doesn't have to be related to you, just part of the wider 'family' or community, and 'of a certain age'.
Many of the names suggested could be thought of like that, I'd have thought. Keep 'Grandpa' or whatever your favourite is for your own Dad, and use a different one for the step grandfather.