Sudden oil pressure drop
 

So I drive a 92 extended cab with The 6.5 turbo I was driving down the road yesterday and I looked down an my oil pressure dropped to 0 so I pulled off an killed the truck when I poped the hood I seen a little smoke coming out of the air filter. When I attempted to restart it all it did was click, my batteries weren't fully charged but they should of had plenty to spin it over so I suspected maybe the starter silonoid might have been bad so I pulled the starter an had it tested an it tested fine. I'm going to fully charge the batteries and put the starter back on an see if it will spin over what's got me is the sudden OP drop any advice would be greatly appreciated.

Was truck ticking or making other strange noises? If not it may be the OPS (oil pressure switch) Replace with AC/DELCO only.

No strange noises or anything like that its been about the most consistent truck I've ever had, the oil pressure as always been 40-45 that's why it struck me as odd an the smoke coming out of the air filter was a new one for me as well