OOM = out-of-memory.
I haven't heard of replacement textures (e.g. ater) causing any memory leakages. Try flying in default scenery and if it doesn't happen, then the scenery addons might well be the issue.

OK, I ran three test flights.
First, in India where the only additions to scenery are dirt runways and water texture.
I flew well beyond the 45 mins of my Christchurch, NZ experience.
Then, I flew in Scotland. Still the dirt runways and water but also a general Northern Europe scenery enhancement.
I flew well beyond the 45 mins of my Christchurch, NZ experience.
Finally, I returned to the Christchurch area where I have more scenery enhancements specific to the Christchurch area...
Program ran out of memory at 45 mins. just like before.
So, it does appear to be a function of how much scenery additions are in an area...
But...
I now have an interesting challenge...
The places I want to fly are where I have the most scenery augmentation but that leads to reduced flight time...
:-/
What to do?