I learned this a couple of years back....

Changed the oil in my 117.968 to 5w50 synthetic from dino, and after a few months, my oil pressure was getting lower and lower when hot.

In the end, after checking everything else, I pulled out the oil filter, and it was nearly clogged! The synthetic oil had lifted all of the crud out of the engine and had plugged up the filter. After that, I changed the filter out every 1000 or so miles until the next oil change. All seems back to normal now.