I have the same car in automatic. I bought it at 99k on the ODO. The previous owner was using caltex formula one 10W30. On my first service at Honda, i used fully synthetic 5W30. It did not cause any problems. I later changed it to Liquimoly Hi-tech 5W40. It is a great oil...i had to replace it at 6000 KM because i replace oil every 6 months eventhough it could have been used for atleast another 6k. I am currently using Caltex fully synthetic (golden pack) it is SN grade 5W40. The engine is in absolutely awesome condition and has NOT reduced a 0.1 pint of oil through seals...
The only oil i would be really careful replacing would be power steering. Those seals leak in an instance.
BTW my civic is now driven close to 120k. Its just a myth that you cannot use fully synthetic in a 100k driven engine.