Oil may be thinning out when vehicle overheats and cannot maintain proper oil pressure
The horn relay on my 86 was taped to the wiring harness on top of the fuse panel.
over full or wrong oil filter
this depends upon your abilities and knowledge. with the right tools and lots of knowledge you can put any engine in the jeep. you may have to do some major modifications depending upon the engine. the big question is, is it worth the time and money.
Replace fuel pressure regulator.
Do you mean a Blazer? There isn't an '86 Tahoe.
usually next to the blinker fluid reservoir unless its turbo then its under the flux compasitor relay unit thingy
It is in the oil pan, you will need to remove the oil pan to service it.
A man named Fredrick Hamilton lived on the grand canyon for 86 years.
it is 86% unsaturated
look along the frame below the drivers door. if not in that area than you should have small inline filters in the engine compartment. most have both types of filters.
That is where you add the oil.