Porridge will always be 1 syn per 20 calories as it contains no free food, so just scan over which ones are less in calories. The pots will always be quite a lot as they have loads in them - you only get 35g plain porridge oats as a HEXB and the pots are often double that.
Here's the Symingtons ones:
Oat Burst Plain Oats Instant Porridge Snack, ambient, 64g pot
12½
Oat Burst Maple Syrup Instant Porridge Snack, ambient, 64g pot
13
Oat Burst Golden Syrup Instant Porridge Snack, ambient, 64g pot
12
Oat Burst Apple & Cinnamon Instant Porridge Snack, ambient, 64g pot
13
Makes Oats so Simple look like lettuce