Re: what version of fortios should I run?
First off, "if it ain't broke, don't fix it". That is, if your FGT runs OK, I wouldn't upgrade it.
Second, v5.6 is still supported until 2021 whereas if you used v5.4 it was about time to upgrade, no choice.
Third, if you decide you'd have to upgrade then IMHO better go with v6.0 than v6.2. Skip the first 4-5 patches of any new major line.
There haven't been any complaints about IPsec VPN with v6.0, and I'm using it daily on several FGTs. Yet, if you use IPS...see "First".
Just my 2 cents.
Ede " Kernel panic: Aiee, killing interrupt handler!"