MOSFET Rds(on) question
\$R_{DS(ON)}\$ is an important parameter, and many datasheets start with mentioning values for them.
For the FDC885N two values are mentioned in the Features section at the start of the datasheet:  


  Max \$R_{DS(ON)}\$ = 27m\$\Omega\$ at \$V_{GS}\$ = 10V, \$I_D\$ = 6.1A
  Max \$R_{DS(ON)}\$ = 36m\$\Omega\$ at \$V_{GS}\$ = 4.5V, \$I_D\$ = 5.3A


From the same datasheet:  

  

So, yes, \$R_{DS(ON)}\$ varies with \$V_{GS}\$, and yes, it's higher at higher temperatures.  

If your manufacturer can't give you the information and you really need it, move on to another manufacturer.