Oh man, to push my late 2008 MBP beyond El Capitan or not. Hmmm.
Was reluctant to do so on an early 2008 Penryn but El Capitan dropped the wifi sometimes. My try with Sierra was a heat machine and I rolled back.
Then I tried High Sierra as a test and after Sherlock got done indexing, it settled down quite nicely. Although it's not supported, DosDude has some lovely tools. You only need to run it again with an OS update to get the ambient light back and make sure recovery is available.
Would say High Sierra is smooth and better than El Capitan but YMMV.
Do it. You can always roll back if you want.
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FWIW, here's where this machine has been. Upgraded to SSD Sandisk 480GB, 6GB RAM.
The optical drive is out with another HDD in. The logic board was replaced by Apple due to the video issue (They tried to get out of the extended warranty insisting on an undoable boot test. I took it back and tricked the machine to boot back on, took a photo with a newspaper that day and then dragged it back to saying, "You told me it was dead so how did I pull this startup off? The genius was angry I had been to two Apple stores and refused to help unless I paid over $300. Called customer relations and they honorably made the repair under the extended warranty for the Nvidia problem. Confirmed that issue in the notes on return. They also replaced the camera.)
Also, I replaced i/o board was replaced after Apple said they wouldn't fix it. (Apparently, some orange soda blew up and got in there unbeknownst to me.) After the little orange cleanup and new i/o board, all good.
The keyboard and top case have been replaced twice. There have been at least four battery changes with the latest last week. The entire display assembly (two lower end cracks) was replaced the other week back as I obtained one Grade A version on ebay for $35.
The machine is used daily and it's running better than ever, knock on wood.
IMHO, we'll never be so lucky as even without Retina, these machines could be fixed and you could educate yourself and do so much of it yourself. Luckily, I've not had to change the logic board myself.