The best option is the thinner manufactured by the brand you use, until you can work out the active ingredients in that & find cheaper alternatives.
However, some brands' thinners are quite a concoction of chemicals & are either very difficult or impossible to replicate, & you will, therefore, be stuck using them.
Mineral turps is OK for Humbrol, but I wouldn't use it in the tin. Even their brand thinners isn't great for that.
Keeping the lid & rim of the tin/jar smooth & undamaged, & clean of paint is your best starting point to prevent the paint drying out; &, when you use it, mix it in the tin/jar & transfer some to a pallet, then seal the tin/jar again while you use the paint on the pallet.