Discovery 300tdi 1997 newbie

Hi peeps proud owner of a discovery 300tdi 1997 auto
I’m a welder n thought I’d get me a little project! Started it 6months ago and now it’s rot free!!! Put it through mot last week luckily it went threw with a few advisories,
So I’ve been driving it for a week, and it don’t like engaging reverse, selects all gear fine, drives fine, but when it comes to reverse it’s very temperamental, no noises nothing! Reverse lights come on but no drive, any help would be much appreciated
Gona drop the oil the weekend

Never had an auto. Read a few threads on LZ about them. Changing the oil seems to make all the difference, so your plan is likely the way to go.

check the oil level.

Engine running upto temp and in Neutral.