If you don't have an on-the-head spring compressor and a way to keep the valves shut, it won't be a fun job to change retainers and locks. You may want to look into a spring compressor that uses a rocker stud as a pivot point. You will also need a way to keep the valve from sliding down while you're trying to assemble.
I would agree that the springs that come with the heads are softer than spec. I don't like 100 lbs of seat pressure on anything with a hydraulic cam. I'd rather see something with 130-140 lbs seat. Remember, springs will lose some of their pressure after some heat cycles and I know that a lot of guys really like to spin their engines up on occasion. Valve float is not a good thing.
As for the
synthetic oil as a break-in, I hope you haven't done any damage to the cam/lifters. If you didn't use any additive or a break-in
oil like Brad Penn or Joe Gibbs, you have really threaded the needle if the break-in has went well. I would be checking the
oil and filter for metal.
As for
synthetic, never, ever use
synthetic oil as a break-in oil. Not only is there no protection for the cam and lifters, chances are it will not let the rings seat either. I never recommend synthetic oil for any of my engines, even after long-term use, and I definitely wouldn't recommend it as an oil to use right after a rebuild/assembly.
Hopefully the low spring pressures that you have helped on the cam break-in. I would change the oil ASAP to a non-synthetic oil.