12 year vs 20 year roll roofing with the same cap sheet. What's the difference?

…as far as longevity?

for example, in a 3-layer Flintlastic system, the 20 year system has a thicker midply sheet, and the 12 year one has a thinner midply. Everything else is the same.

Is the manufacturer suggesting that one system will last 8 years longer than the other?

Since both systems have the same capsheet, shouldn’t their realworld lifespan be about the same?

Put Apoc Armor base and Armor Flex white silicone over the cap sheet and it will last till your grandchildren are old. The Silicone will also protect all ponding areas and will drop the surface temperature more than any other coating.