Hi,
My oil pressure gauge comes on when I start my disco 1 but then it goes away in about 5 seconds. Is this normal?

 

Its not. You have low oil pressure at start up. This could be caused by:
1. Wrong oil, try 15-40 Rottella it will probably help
2. Worn out main, rod or camshaft bearings.
3. Partially clogged oil pick up tube.
4. Worn out oil pump gears and/or timing cover.
5. A small oil leak at the oil lines, oil filter gasket, timing cover or else where causing the oiling system to drain when the engine is off. This will make it take a little extra time to recharge the system and turn that light off.
I believe that means that your pressure is below 10 psi. I am willing to bet that this happens more noticeably after it has sat over night. Most often it means that you need new rod and main bearings. What year and how many miles?

 

I think it might be my oil. I was running 20W-50 during the summer months.

 

Running thicker oil will help. Help is the key word here. 20-50 is thicker than 15-40 and will bring up your oil pressure but may not shut that light off right away either. Especially on start up. I run 10-30 in mine now and my oil light doesn't even flicker because I rebuilt the motor. I rebuilt it in part because my oil light was staying on for 10-15 seconds on cold start up. I fixed that (mostly) by replacing the main and rod bearings which wasn't easy but doable with the motor in place before I did the full rebuild.

 

I have everything to replace my oil pump, but I guess I'll replace the main and rod bearings while I'm at it. I guess this will help until I have to do a complete rebuild. It's a 1999 Disco 1 with about 150,000 miles

 

Besides the oil pump it could just be a clogged or loose oil pickup tube. You'll want to drop the oil pan first and see what's going on. Cleanout the screen on the pickup tube and check it is tight and sealed against the block. Clean the gasket surfaces and install a new gasket of your choice (I prefer Right Stuff gasket maker). Refill with oil appropriate for your climate. 10W- for cold climates, 15W- for warmer climates or summer. I wouldn't run 20W- unless I was in Arizona or Mexico.

If the issue isn't solved at that point, then start looking at more difficult/costly fixes like the oil pump and bearings.

 

running 10-30 rotella
1-2 sec when 45F or higher
about 4 to 5 this morning at 32F

I have new oil cooler lines on my 122k engine the rest of the oil system is orig
the light would stay on longer before I replaced my cooler lines (which is why I replaced them)

update:
just ran out to the store a min ago - truck was cold again (sat 7hrs+ - )
36F took about 3.5 sec