Iv'e had my 1998 discovery 4.0 GEMS for two years now , about a month after i parked it the alternator stopped making charge so i hooked up a battery maintainer and have let it sit, starting it up every few months. Well this morning i decided it was time to start it up as it had been at least 2 months since the last start , after running a few minutes as it gets warm i give it a few revs , only up to 1500, except its exactly that , i watched the tach move which it never did when it had a bad alternator even with the batter charger hooked up, but i see when it goes down too idle , the oil light comes on and any revs at all above the notch before the 1 on the tach and it will go out, now its worth mentioning that it had an oil change days before i finally parked it, and that it is 5-30 but it never did this before with the oil. i am a student and only 19 with limited tools, i dont even have a full socket set as they were my grandfathers and many are missing (i do have a 20 pack of 10mm though) im just wondering what this could be, its worth noting i received the truck free from a freind and it was probably not treated well for the few months they had it, there are a number of weird things as well ,such as a 6 pin towing connecter on the front metal bumper and in the passenger tailight housing any input would be helpful! Iv'e also heard that the iol pump can go bad if its not started every few months but i hope thats not the case

 

If I'm reading your post correctly the oil in the truck is two years old. An oil and filter change is the first order of business.

 

The Tach is run off the alternator. I agree with WaltNYC, get a good oil change and go from there.

 

Change out the oil with Rotella 15-40