Just because the 8th digit and engine code is the same doesn't mean the engines are. The 8th digit for LMG never changed for example yet the engine did. So much so that there are cam profile and power differences as well as variants with and without VVT. You need to get the proper year range of engine if you don't want to deal with compatibility issues. Its why yards and even sellers on ebay list specific year ranges with the engines.
The 8th digit can make all the difference by year. J for 2007-2009 was a 5.3. For 2010 it changed to 6L hybrid. Then it changed again to 6.2L. Without knowing the year you don't know what engine you're dealing with only going by the 8th digit.
ok, well your throwing hybrids into the mix, of course there's going to be a difference and it's not a 5.3
if you call a yard and say i'm looking for a vin j 5.3l there not going to give you either one of the examples you gave above.
there are only 26 letters in the alphabet and 0-9 of course there's going to be repeat codes over time
same if you were looking for a vin j 6.2l (that's a l86) that started with the k2's, completely different generation engine, unless your a dumbkoff and just completely ignore what you are ordering then I guess you get what you get, common sense has to kick in at some point
if you were to call and order a motor for a 2000 yukon you could end up with the completely wrong motor
same if you ordered one for a 2007 silverado
or anything else that they changed mid-year or produced both a old style and a new style, for instance a 2019 silverado LD (last design, technically it's a 2018)
of course the year is going to matter but more importantly the engine size
that's why I bought my 2012 because I know I keep my vehicles for a long time and I know the 2012 has one of the widest availability of crossover parts
had I bought a 2007 I knew I would be limited to 2 years of crossover engine/transmission and 3 years on some interior parts
with my 12 I have 5 years engine/transmission and 4 years interior
it was a no brainer for a poor person like me that might not have the money buy a new transmission when it goes out, having a wide range of available used parts is a wise decision