My experience is that oatmeal porridge first thing in the morning is that it raises my blood sugar quite a bit - from say 5.1 to 8.5 or even 9.5 at one hour. If I eat it in the evening it might add +1.5 or +2. I have now basically decided to avoid it.
I first realized that I had a issue with glucose intolerance when I had a blood test for something else and a (non-fasting) glucose was also taken at the same time. I had eaten a big bowl of oatmeal porridge with apple and dates about an hour before. My “random” glucose came back at 8.5. That was quite a shock since my fasting glucose previously had been in the upper 4s to mid 5s and my A1c was 5.0%.
I am trying to figure out how to eat low carb enough to keep my blood sugar from spiking over 6.5 most of the time and never over 7.8 while not losing any weight. (I need to gain weight.) If I have eaten a carb and checked at one hour and my blood sugar is over 7.8 then it goes on a black list of things I just shouldn’t eat - even if it is much lower at 2 hours. I check at one hour because I find that when eating carbs my blood sugar is higher at one hour than at two hours. My understanding is the really short peaks like at 20-30 minutes aren’t that relevant for health but elevated blood sugar for more than an hour starts to do damage.
Interestingly porridge and sweet potatoes that have the reputation to be “healthy” cause me problems, but a dense whole grain and seed bread doesn’t seem to raise my blood sugar much - and everyone says avoid bread.