I think the best thing you could do right now is go over the truck with a fine tooth comb. You asked the right question about oil, now check the other fluids. Most people are pretty lax about upkeep, I know I was before I learned. So;
-Get a small book to keep track of all your maintenance, it will help you save money. Keep it in the glove box. Alternately, use a felt and write what you did and when somewhere under the hood. On the rad support is what I use. Example; 10w30 Feb/2019
-Rad fluid should be changed out every couple years. The rust inhibitors get used up even if the truck is sitting.
-The brake fluid should be flushed out every few years. Brake fluid likes to absorb moisture from the air and then stuff fails. Start at the tire farthest away from the master cylinder. That will be the passenger side rear. Then you go in order of next farthest and so on. It's an old truck, there is the likely hood of twisting off a bleeder valve. Better now than when you need it for work.
- Top up the windshield wiper fluid.
- If it's an automatic, warm up the engine for 10 minutes in park and check the dipstick while it's running.. It should be a clear red. You could also check before running and put a slight mark on the dipstick, this way you will know where it should be when cold and not running.
- If 4x4, check the transfer case fluid.
- Find out how to check and change the differential fluid. The front too if 4x4.
- Check all the u-joints. Make sure the truck is blocked so it can't roll back or forward. Put the trans in nuetral and get under and see if you can twist the drive shaft back and forth compared to the opposite of the shaft, [the u-joint is in the middle of your two hands]. If there is ANY movement back and forth change the u-joint. If 4x4 there are two more u-joints on the front drive shaft and one on the drivers side drive axle and two on the passenger side axle
- Check your brakes. Take the drums off and look at the shoes, there should be at least 1/8" of friction material left. On the front you can see into the caliper by removing the tire. also should have at least 1/8" of lining left. The front wears out twice as fast as the back.
- Check over your front end and steering. Have somebody in the truck slowly turning the front wheel back and forth. Look at all the joints, there should be no movement between two pieces in relation to each other. Check the steering column for slop too. The rag joint can disintegrate.
Anything I forgot I'm sure someone else will help.