Do I need to optimize all posts for seo?

I can't found this answer, so I will ask here, hopefully, a good person will ask me.

If I have a blog with hundred of blog posts (that i am not interested in rank them, but are important to my site) that are not SEO friendly: 20-30 words, no focus keyword etc

And I want to add quality content with SEO optimized articles, is this ok from SEO point of view? Will non-seo articles have bad impact for my good seo articles and my website in general?

Or should i transform non-seo blog posts to products?

Thanks!

short answer: yes you need quality content and it should be long

I would recommend building them out even if they don't have the same value as an SEO optimized article. They can still provide value to the people that visit your site.

Optimising all your posts is good for your blog.. Good quality content will always increase traffic.

Actually, a properly optimized content is a clear indication that the site isn't a spam site (splog), and it would contribute brownie points when it comes to ranking on the SE.
Thus, although you do not want to rank for the other posts, perhaps you can automate the process, by installing a plugin that can optimize your seo automatically for you, so that it will eventually be a fire and forget thing. Maybe yoast would be able to help you automate your seo partially.

You can always succeed in SEO and IM in general if you put yourself in the shoes of the search engine bots as well as the readers. Those non-seo articles will definately add some juice to your ranking efforts so by all means do it! the good thing is you dont have to do it all at once, you can always edit bit by bit as you channel most of your energy on the actual SEO Content. All the best!