The 6.2 in my '82 GMC has only refused to start when cold maybe 3 or 4 times,it has surprised me many times when it was below 20 above and started without having the block heater plugged in ,I just had to cycle the glow plugs a few times and it eventually started--the few times it failed to run,it did fire up and stall a few times,then the batteries wore down and wouldn't spin it over fast enough..it sounds like hell though,until it warms up some..like the pistons are .030 too small for the cylinders!..
I notice it has started much better and turns over faster since I started adding thinner motor oil to replenish what leaks out of its rotted oil pan--I also put a gear reduction starter in it which might have helped too,but I think the oil viscosity made a bigger difference..
The 6.2 in my '85 Burb starts pretty hard,I have replaced 4 out of the 8 glow plugs (drivers side) but the other side might still have some dead ones...the engine in that is original and has around 155K on it,and it is much quieter than my other 6.2,but it sounds like it doesn't have as much compression when you spin it over--it has a gear reduction starter also...
I managed to get it started the other day using just one battery,when it was close to 60 degrees out,after sitting a good 6 months ,the day before I tried starting it when it was about 50 degrees and it did fire up and run a few seconds,then the battery was too low to spin it fast enough..the next day after recharging the battery it fired up almost instantly...but I saw smoke at the battery,and found out the small ground wire going from the ground cable to the radiator support had fried--found out the ground cable was loose where it bolts to the alternator bracket..

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I guess when it had both batteries hooked up ,the other ground cable was doing most of the "work",without the other battery,that small ground wire tried carrying the whole current load,and "whoof"....glad it didn't melt into any other wires or things near it...
If you dont plan to tow or use it heavily during the winter,I bet using 10W-30 oil in your 6.2 will make it start better...15W-40 turns into paste below 20 degrees...you need it to spin over as fast as possible to get it fired up in the extreme cold..using a block heater is a good idea too..