Sometimes you will hit a zone it doesn't make any difference what kind of cement you use, the zone will just drink it. The pressure from the production zone will push the cement into this zone and basically there is no cement left to seal the casing.
I don't know enough right now to know where the oil is coming in. Is it coming up through the production casing or is it going around the production casing and coming up through the intermediary string? Don't know.
I've been using 4350 in 30-06, .308, 7X57, and 8X57.