Turns over but dont start let off key and sputters 3or4 times 1998 Vortec 350 Chevy Z71 4x4

I have a 96 Chevy 4x4 Z71 with the vortec 350. I just bought the truck and it had a leaky freeze plug. The truck ran but I had it towed home anyway. I replace the freeze plug, just drove it for the first time and it drives great, runs strong, everything is good including the running temp. I put fresh oil and filter and brand new antifreeze in it. When I opened the radiator cap to make sure the level was right, there was oil in the water (unlike the other guy you helped that had water in the oil). I am very perplexed. The new green antifreeze appears brown now and when I felt it, it is clear there is oil in it. I didn't drain the entire system when I changed the freeze plug, just added the antifreeze. But when I took the freeze plug out, the water that came out appeared pretty clear (can't say for sure, but I didn't think there was oil in it). Any help is appreciated. Thank you, Tony tonmar@rsaa.com turns over but does not start. when u let off key it sputters like maybe 3or4 times 1998 chevy Z 71 5.7 350 vortec