Hello,
Which release do you use? From where did you get the original oil_flot subroutine you use? Why did not you start from the standard oil_flot subroutine available in the $HOMETEL/sources/telemac2d folder?
I think your issue comes from the mismatch between the number of arguments of your version of oil_flot subroutine and what is expected by TELEMAC-3D. Indeed, compared to oil_flot described in interface_telemac2d module, there is no expected AT variable. You may get trouble with your simulation for this reason I think (I have just tried to delete this additional variable in the arguments and declaration of the subroutine and the computation goes further).
Anyway, in 3D, do not forget to also define the elevation where you release oil particle (it is then written in the log that the oil particle is put at the surface otherwise).
Hope this helps,
Chi-Tuan