are the lifters ticking? If you still have a quiet engine it's more likely a gauge or sender unit issue. The engine will talk to you if it needs oil. I had the sender go bad on my '99 at about 130K miles, it started reading lower and lower then nothing, no noise, and when I pulled the sender I noticed the plug moved relative to the body of the sending unit. The short of it though is to listen to the lifters.

Sean

99 XJ Sport 2 Door, 4.0L, 5 speed, all stock. Daily Driver
79 F150 Standard Cab Short Box, 400ci, NP435, NP205, 35" SSRs