stepAIC formula upper limit guidelines

Hello,

I am attempting to refine an lm()-generated model using the stepAIC 
function.

My model has approximately 20 inputs and I am trying to determine the best 
upper limit scope for using those inputs.

My lower limit is "y ~ 1" and my original upper limit was "y ~ x1 + x2 + ... 
+ x20".

This is great start, but I am wondering if some other (more broad) upper 
limit would allow for combinations of inputs (examples: "x1*x3" or "x5/x7") 
before the coefficients are applied.

Specifically, I am trying to determine what syntax will give stepAIC the 
most flexibility to slice and dice my inputs as it searches for the best 
possible model.

Any details or references would be immensely appreciated.

Thank you,
Tate Avery