Even if 10°C difference, that does not mean the inferior one is bad. Would you be unhappy with a Ferrari because it is "not that good" compared to the Bugatti? Any TIM (thermal interface material) is better than no TIM or even good TIM improperly applied. Just because, 40°C is nice and cool, that does not mean 50°C is too warm. It is in fact, nice and cool too. If you are not doing extreme overclocking, the case interior is clean of heat-trapping dust, and, as eidairaman1 noted, you have set up case cooling properly, even plain old OEM thermal pads will provide sufficient transfer of heat to the OEM cooler.
However, 32.9°C (91.22°F) is quite warm. It is important to remember blowing hot air on a hunk of metal just causes that hunk of metal to heat up faster. A fan cannot cool a hunk of metal cooler than the temperature of the air it is moving.
"A dirty pad" is the key phrase there. Dirty pads don't just happen over time. Your example just illustrates what happens if the cured bond is somehow broken, or the TIM as not applied properly in the first place. It has nothing to do with brand of CPU, or just the fact the TIM is X number of years old. So I stick with my original comment and assuming the TIM is applied properly, there is NEVER any need to pull the CPU and replace the TIM unless the cured bond was broken. It will easily last 10, 12 years or longer. Even if it dries out, the solids left behind are still doing their job of occupying the microscopic pits and valleys in the mating surfaces, preventing insulating air from getting in there.
So how does the bond break? Often, ironically, by the twisting the heat sink to see if it is still tight!
But it can also happen by getting knocked about during transport. This is particularly so with tall coolers.