My 99 4.6 HSE has 100k miles. The previous owner used straight 10w30 oil. Is it too late to move to synthetic or synthetic blend? Or should I continue using regular motor oil?

 

What state do you live in, do you have freezing winters?
10W/30 is fine, you can go heavier if you live in the 90 to 100 degree states, and if you want to switch to a blend or synthrtic, it is never too late.
I would suggest using a partial synthetic, I use Castrol's 10W/30 here in Colorado, used to use their 20W/50 when I live and played in the high deserts of California.
Two thoughts for you, do a good engine flush to clean out the engine and make sure to use good quality oil filters like the K&N 3001 or a Mobil 1 filter.
Have fun,
Mike