Adsense is encouraging me to turn on Auto Ads. Currently my web site has manual ads showing at specific places on the pages, using Adsense code, such as 'leaderboard'. If I turn on auto ads, should I remove the manually placed ads?
No, it's not necesary. AdSense detects the presence of other ad units before creating it's auto ad placements.
Have you manually placed ads on your page? There’s no need to remove them if you don’t want to. Auto ads will take into account all existing Google ads on your pages.