and it's ok to use it on the non-enamelled oven surface, like the one OP describes we have too - gritty surface
To ensure we're on the same page, I assume you mean something like the light grey behind this side rack?
The product can't be used on that nor should anything. They burn the grease when heated above 200c so should for the most part he self cleaning.
The issue most people have when they clean ofens themselves is they just straight up put product in and expect the grease to disappear.
The step being missed is the most important one, which is to first remove the burnt on grease as much as possible use NG a flat blade, like the one linked below.
HEAVY DUTY SCRAPER Fixed blade heavy duty blade holder with removable safety cover.The more robust longer lasting alternative to our standard single-edge blade holders.This scraper will not click in like standard blade holders and have a longer handle
dirtbusters.co.uk
(this can be bought from amazon but I can't quickly find it listed some reason)
This removes all the big stuff, leaving, for the most part, just a small thin amount of grease that the stronger product can easily then deal with. No matter how strong of a product you use inside the oven, without first scraping the big stuff off with a blade, you'll still be left with big chunks around. The scrapers are so good, you can often get away without having to use any strong chemicals, just a good cream cleaner/degreaser can often get you satisfactory results.
I hope this helps.