oil filter

So here we go for my 2nd forum post ever lol I just purchased my 07 Nitro SXT 4wd from a previous owner... and today my oil light came on.. but only when my rpms dropped below 1000 h once its over 1000rpms it turns off...I just replaced the battery today also bcuz I took it to get tested n it...

Good morning guys, Got a few questions. I just picked up a nitro and am looking to bring it back to 100%. 1.Oil change on 3.7 whats the secret. The filter is in the dumbest spot. 2. It has 90,000 on it and i want to change transmission fluid. I see it says its good for life but do you think...