Gear selection problem is often due to the shaft sticking in the top hat bushes at the gearbox end. Good "How To" guide for the 147 will be the same for the 156. I would avoid coppergrease though as it will speed up the aluminium corrosion and it might damage the plastic bushes. If you feel the need to use grease then a silicon based grease like the plumbers use might work.
I use aluminium based grease similar to coppergrease for my alloys to avoid any corrosion issues.
The bushes are not listed a separate part for the GT but if you order Part number 0046467103 Description BUSHING you will be sorted. Come in packs of 5 which is stupid, as you would obviously replace them in pairs. Only £1.40 each though.
"When you sit in a Focus RS or a Golf R32 itís like sitting in a commercial for Lynx aftershave. When you sit in an 147 GTA itís like sitting in a Venetianís hand-made suitcase." J.Clarkson 2003.
Current fleet: 156 Sportwagon jtd 16V (facelift) 2.4 Spider, Leon Cupra SC
Projects: Spider S4, Matra Murena 2.2