Not 100% sure. Replaced o ring with the fatter, idiot proof orange one and no issues at all. Crews Chevy put the motor in and used a blue o ring. Had decent pressure. PCM removed blue and replaced with blue and had questionable pressures. I used the orange one which is a little fatter and narrower inside diameter and all is good.
Crews mechanic even told me he pulled an orange one off old motor but GM provided them with a blue one in his parts "kit."
Oh well all is good now and pretty painless work.
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