I will talk about the major issue, which is overheating, you could kill the engine.
Stop driving until you check it out and resolve it. But welcome to chaos manor.
1. Normal temp tange on the guage is 8:00 - 9:00. Where you saw it is the shake and bake zone - you bake it, then shake out the contents of your wallet. You are overheating, and the oil light came on at 7PSI because the oil was so thinned out from over heating. This is almost time to call the tow truck serious.
2. Be sure to download the RAVE tech manual set, it is free, covers shop, electrical, body/interior and owners manual. Tech section of our Discovery 1 forum has work arounds for many problems, new2me high mles service list, etc.
3. I will copy into this space the reply I just gave someone else on overheating. From what you have said, I think your radiator could be the main issue, or water pump. You are so marginal on cooling that just the extra from AC on puts you over the edge.
Let's cover all the bases. Not in order.
1. It might be the radiator, because on the highway you have a lot more air going through it. Check coolant jug level, and look in the space between the radiator and the condenser, and remove any build up of leaves and trash. Check radiator fins and condenser fins for gobs of mud, bugs, etc. Wash out with garden hose, but not pressure washer which will tear fins. You can take off the plastic plug on the top of the radiator and look inside with a flashlight, may see calcium build up on radiator tubes. Radiator is brass and copper. Even if you don't see it it may be present deep inside. Throw away plastic plug and replace with $2 brass 1/2 inch NPT plug and teflon tape. If radiator is really suspect, you can have a small indy radiator shop "boil it out" with hot acid, and "rod it out" (push small rods through tubes and knock out calcium) for about $65. A blocked up radiator won't cool, and also reduces air flow to the fan clutch. Fan clutch has to see 170 degrees on the face of it to start engaging to full power.
2. Test clutch fan when cold, not running. Should not spin, should only turn a small fracion of a revolution. Crank up, fan should "roar" from air flow, mine moves enough air to spin the electric fans even though they are not on. Wait until truck warms up. Roar goes away normally. Turn off truck. Spin fan, should not free wheel, and should not turn more than 1/2 revolution or so. Bad clutch is big problem with overheating, technical section has write up on conversion with $49 Chevy unit.
Fan clutch replacement Note - cupped side of fan blades toward engine block.
3. Electric fans should come on whenever AC is turned on. If they don't, you will have poor AC cooling in traffic. They can also come on when coolant hits 212, or when coolant is even hotter and fuel temp is 70C and truck is turned off, in which case they will run for about 10 minutes. A wierd problem happens if someone reverses the wires to those fans, they blow out the grille, subtracting air from the radiator instead of adding it, making overheat extra fast. Test fan direction by switching on truck, but don't crank, turn on AC and inside fan. Condenser fans should come on, and a wimpy plastic bag held by the grille should be pulled toward grill, not blown away.
4. In the technical section you will find a link to proper belt route. Be sure to check this out.
Discover Defender Maintenance Tips, Accessories, Parts & More at Roverparts.com
5. I bought my truck and had belt route wrong, ac condenser fans wired backwards, radiator clogged up (PO had never used distilled water), wimpy thermostat, water pump started leaking, and fan clutch so bad it free wheeled. You might have a thermostat problem, but usually that does not show up just when AC is on and driving slow.
6. After I worked on all that, mine will idle and temp guage stays about 8:00 psoition. Over 9:00 is too warm. A scanner that can read live data will show you how bad things are getting.
7. The water wetter product at the auto parts stiore does work to drop temps while you await repairs.
Note: Pix show normal temp guage reading, but it should be 195 temp, I was at 207, imagine what yours was at the top of the guage. Also shows what to expect from a water pump that is wobbly or grinding, note coating from use of "stoppz leekz", and the replacement fill plug.
New Owner 99 Discovery1 SD...oh the problems
