Scoping of private variables must be declared explicitly with OpenMP. With Sun directives, the compiler uses its own default scoping rules for variables not explicitly scoped in a PRIVATE or SHARED clause: all scalars are treated as PRIVATE, and all array references are SHARED. With OpenMP, the default data scope is SHARED unless a DEFAULT(PRIVATE) clause appears on the PARALLEL DO directive. A DEFAULT(NONE) clause causes the compiler to flag variables not scoped explicitly. However, see 4.4 Some Tips on Using Nested Parallelism for information on autoscoping in Fortran.
Since there is no DOSERIAL directive, mixing automatic and explicit OpenMP parallelization may have different effects: some loops may be automatically parallelized that would not have been with Sun directives.
OpenMP provides a richer parallelism model by providing parallel regions and parallel sections. It could be possible to get better performance by redesigning the parallelism strategies of a program that uses Sun directives to take advantage of these features of OpenMP.