What product to use on Hood Hinge springs?

As above, yes you can through the splash shield. If I had car that I was not concerned about nicking the paint I would just take the fender off (although removal, replacement reallignment takes time). If I had nice paint, or at least paint I was trying to keep as nice as I could, it's worth it to at least try to do the job through the splash panel. Tire and wheel off, point the wheels away from the side you are working on, take a screw driver and break off the old push ring. Looking at your avatar, this would be the time to replace the splash shields with some plastic ones off a 73 and later Dart or Demon - they weigh a lot less and never rust - direct replacement.