The 918+ is not sold in a variant with 8GB of RAM, though you can easily upgrade it. The 1019+ does come with 8GB of RAM (and an extra drive bay.)
The guide @ChuckPa linked to is a good one to follow. However, the article (Synoguide) has some incorrect information and one thing I disagree with.
Synoguide states that the 918+ comes with two 2GB chips of RAM. This is incorrect, it comes with one 4GB chip. If you want to upgrade it to 8GB total, you just need to install a second 4GB chip.
I use this one from Crucial: https://www.amazon.com/gp/product/B01775JF9M/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
What I disagree with is Synoguides recommendation to use 16GB of RAM. This has been a heavily debated topic with no real right or wrong side. My stance, regardless of what the system reports, Synology states the 918+ only supports 8GB of RAM, Intel states that CPU only supports two channels of 4GB (8GB of RAM.)
Technical arguments aside, If you are only going to use this for Plex, 8GB of RAM is more than enough.
And… $25 (USD) for another 4GB chip is a cheaper upgrade than $105 for 16GB that you may or may not see the full benefit of. Otherwise, if you are going to pay $100 for a RAM upgrade, that money would be better spent on the Synology 1019+ to increase your drive count (and you get 8GB of RAM.)
