I've been following http://www.minecraftforge.net/wiki/Create_a_Fluid yet right clicking my fluids' blocks with a bucket returns a water bucket.

 

Fluids' block class (they're all the same except the names changed):

package com.hlaaftana.mods.OlivicBlocks.blocks;

import com.hlaaftana.mods.OlivicBlocks.help.Reference;
import com.hlaaftana.mods.OlivicBlocks.init.ModFluids;
import com.hlaaftana.mods.OlivicBlocks.tabs.ModTabs;

import cpw.mods.fml.relauncher.Side;
import cpw.mods.fml.relauncher.SideOnly;
import net.minecraft.block.material.Material;
import net.minecraft.client.renderer.texture.IIconRegister;
import net.minecraft.util.IIcon;
import net.minecraft.world.IBlockAccess;
import net.minecraft.world.World;
import net.minecraftforge.fluids.BlockFluidBase;
import net.minecraftforge.fluids.BlockFluidClassic;
import net.minecraftforge.fluids.BlockFluidFinite;
import net.minecraftforge.fluids.FluidStack;

public class BlockFluidOliveOil extends BlockFluidClassic {
public BlockFluidOliveOil(){
	super(ModFluids.oilOlive, Material.water);
	setBlockName("oilOlive");
	setBlockTextureName(Reference.MODID + ":" + getUnlocalizedName().substring(5));
	setCreativeTab(ModTabs.tabOlivic);
}
@SideOnly(Side.CLIENT)
protected IIcon stillIcon;

@SideOnly(Side.CLIENT)
protected IIcon flowingIcon;

@Override
public IIcon getIcon(int side, int meta){
	return (side == 0 || side == 1)? stillIcon : flowingIcon;
}

@SideOnly(Side.CLIENT)
@Override
public void registerBlockIcons(IIconRegister register){
	stillIcon = register.registerIcon(Reference.MODID + ":oilOlive_flowing");
	flowingIcon = register.registerIcon(Reference.MODID + ":oilOlive_flowing");		
}

@Override
public boolean canDisplace(IBlockAccess world, int x, int y, int z){
	if (world.getBlock(x,  y,  z).getMaterial().isLiquid()) return false;
        return super.canDisplace(world, x, y, z);

}

    @Override
    public boolean displaceIfPossible(World world, int x, int y, int z) {
    	if (world.getBlock(x,  y,  z).getMaterial().isLiquid()) return false;
    	return super.displaceIfPossible(world, x, y, z);
    }
}

 

BucketHandler:

package com.hlaaftana.mods.OlivicBlocks.handlers;

import java.util.HashMap;
import java.util.Map;

import net.minecraft.block.Block;
import net.minecraft.item.Item;
import net.minecraft.item.ItemStack;
import net.minecraft.util.MovingObjectPosition;
import net.minecraft.world.World;
import net.minecraftforge.common.MinecraftForge;
import net.minecraftforge.event.entity.player.FillBucketEvent;

import com.hlaaftana.mods.OlivicBlocks.init.ModBlocks;
import com.hlaaftana.mods.OlivicBlocks.init.ModItems;

import cpw.mods.fml.common.eventhandler.Event.Result;
import cpw.mods.fml.common.eventhandler.SubscribeEvent;


public class BucketHandler {

        public static BucketHandler INSTANCE = new BucketHandler();
        public Map<Block, Item> buckets = new HashMap<Block, Item>();

        private BucketHandler() {
        	BucketHandler.INSTANCE.buckets.put(ModBlocks.tar, ModItems.bucketTar);
        	BucketHandler.INSTANCE.buckets.put(ModBlocks.creosote, ModItems.bucketCreosote);
        	BucketHandler.INSTANCE.buckets.put(ModBlocks.oilOlive, ModItems.bucketOliveOil);
        	BucketHandler.INSTANCE.buckets.put(ModBlocks.oilOlive, ModItems.bottleOliveOil);
        	MinecraftForge.EVENT_BUS.register(BucketHandler.INSTANCE);
        }

        @SubscribeEvent
        public void onBucketFill(FillBucketEvent event) {

                ItemStack result = fillCustomBucket(event.world, event.target);

                if (result == null)
                        return;

                event.result = result;
                event.setResult(Result.ALLOW);
        }

        private ItemStack fillCustomBucket(World world, MovingObjectPosition pos) {

                Block block = world.getBlock(pos.blockX, pos.blockY, pos.blockZ);

                Item bucket = buckets.get(block);
                if (bucket != null && world.getBlockMetadata(pos.blockX, pos.blockY, pos.blockZ) == 0) {
                        world.setBlockToAir(pos.blockX, pos.blockY, pos.blockZ);
                        return new ItemStack(bucket);
                } else
                        return null;

        }
}

 

Custom buckets' register:

package com.hlaaftana.mods.OlivicBlocks.init;

import net.minecraft.init.Items;
import net.minecraft.item.*;

import com.hlaaftana.mods.OlivicBlocks.init.ModArmory;
import com.hlaaftana.mods.OlivicBlocks.help.RegisterHelper;
import com.hlaaftana.mods.OlivicBlocks.items.*;

import cpw.mods.fml.common.IFuelHandler;

public class ModItems{
*A bunch of items*
public static Item bucketTar = new OlivicBlocksBucketItem("Tar", ModBlocks.tar);
public static Item bucketOliveOil = new OlivicBlocksBucketItem("OliveOil", ModBlocks.oilOlive);
public static Item bucketCreosote = new OlivicBlocksBucketItem("Creosote", ModBlocks.creosote);
public static void init(){
	*A bunch of items*
	RegisterHelper.registerItem(bucketOliveOil);
	RegisterHelper.registerItem(bucketTar);
	RegisterHelper.registerItem(bucketCreosote);

	RegisterHelper.registerFluidContainer(ModFluids.tar, bucketTar);
	RegisterHelper.registerFluidContainer(ModFluids.oilOlive, bucketOliveOil);
	RegisterHelper.registerFluidContainer(ModFluids.creosote, bucketCreosote);
	RegisterHelper.registerFluidContainer(ModFluids.oilOlive, bottleOliveOil);

 

The RegisterHelper class:

package com.hlaaftana.mods.OlivicBlocks.help;

import cpw.mods.fml.common.registry.GameRegistry;
import net.minecraft.block.Block;
import net.minecraft.init.Items;
import net.minecraft.item.Item;
import net.minecraft.item.ItemBlock;
import net.minecraft.item.ItemStack;
import net.minecraftforge.fluids.Fluid;
import net.minecraftforge.fluids.FluidContainerRegistry;
import net.minecraftforge.fluids.FluidRegistry;

public class RegisterHelper {
public static void registerBlock(Block block){
	GameRegistry.registerBlock(block, block.getUnlocalizedName().substring(5));
}

public static void registerItem(Item item){
	GameRegistry.registerItem(item, item.getUnlocalizedName().substring(5));
}

public static void registerFluid(Fluid fluid){
	FluidRegistry.registerFluid(fluid);
}

public static void registerFluidContainer(Fluid fluid, Item bucket){
	FluidContainerRegistry.registerFluidContainer(fluid, new ItemStack(bucket), new ItemStack(Items.bucket));
}
}

 

OlivicBlocksBucketItem:

package com.hlaaftana.mods.OlivicBlocks.items;

import net.minecraft.block.Block;
import net.minecraft.client.renderer.texture.IIconRegister;
import net.minecraft.init.Items;
import net.minecraft.item.Item;
import net.minecraft.item.ItemBucket;

import com.hlaaftana.mods.OlivicBlocks.help.Reference;
import com.hlaaftana.mods.OlivicBlocks.tabs.ModTabs;

import cpw.mods.fml.relauncher.Side;
import cpw.mods.fml.relauncher.SideOnly;

public class OlivicBlocksBucketItem extends ItemBucket{
public OlivicBlocksBucketItem(String name, Block fluid){
	super(fluid);
	setCreativeTab(ModTabs.tabOlivic);
	setUnlocalizedName("bucket" + name);
	setContainerItem(Items.bucket);
}

@SideOnly(Side.CLIENT)
@Override
public void registerIcons(IIconRegister par1IconRegister){
	itemIcon = par1IconRegister.registerIcon(Reference.MODID + ":" + getUnlocalizedName().substring(5));
}
}

Main class:

package com.hlaaftana.mods.OlivicBlocks;

import net.minecraft.item.ItemStack;

import com.hlaaftana.mods.OlivicBlocks.crafting.Recipes;
import com.hlaaftana.mods.OlivicBlocks.generation.OlivicBlocksWorldGen;
import com.hlaaftana.mods.OlivicBlocks.generation.VanillaChestLootGen;
import com.hlaaftana.mods.OlivicBlocks.handlers.ConfigHandler;
import com.hlaaftana.mods.OlivicBlocks.handlers.FuelHandler;
import com.hlaaftana.mods.OlivicBlocks.help.LogHelper;
import com.hlaaftana.mods.OlivicBlocks.help.Reference;
import com.hlaaftana.mods.OlivicBlocks.init.ModBlocks;
import com.hlaaftana.mods.OlivicBlocks.init.ModFluids;
import com.hlaaftana.mods.OlivicBlocks.init.ModItems;
import com.hlaaftana.mods.OlivicBlocks.init.OreDictionaryRegister;

import cpw.mods.fml.common.IFuelHandler;
import cpw.mods.fml.common.Mod;
import cpw.mods.fml.common.event.FMLInitializationEvent;
import cpw.mods.fml.common.event.FMLPostInitializationEvent;
import cpw.mods.fml.common.event.FMLPreInitializationEvent;
import cpw.mods.fml.common.registry.GameRegistry;

@Mod(modid = Reference.MODID, name = Reference.NAME, version = Reference.VERSION)
public class OlivicBlocks {
OlivicBlocksWorldGen eventWorldGen = new OlivicBlocksWorldGen();
IFuelHandler OBFuelHandler = new FuelHandler();
@Mod.EventHandler
public void preInit(FMLPreInitializationEvent event){
	LogHelper.info("Pre-init starting, registering fluids");
	ModFluids.init();
	LogHelper.info("Fluids registered, registering items");
	ModItems.init();
	LogHelper.info("Items registered, registering blocks");
	ModBlocks.init();
	LogHelper.info("Blocks registered, initalizing world generator");
	GameRegistry.registerWorldGenerator(this.eventWorldGen, 0);
	LogHelper.info("World generator initialized, registering OreDict entries");
	OreDictionaryRegister.registerOres();
	LogHelper.info("OreDict entries registered, checking/creating config");
	ConfigHandler.initProps(event.getModConfigurationDirectory());
	LogHelper.info("Config created/checked, adding loot");
	VanillaChestLootGen.init();
	LogHelper.info("Loot added");
	LogHelper.info("Pre-init done");
}
@Mod.EventHandler
public void Init(FMLInitializationEvent event){
	LogHelper.info("Init starting, registering fuels");
	GameRegistry.registerFuelHandler(OBFuelHandler);
	LogHelper.info("Fuels registered, adding recipes");
	Recipes.init();
	LogHelper.info("Recipes added");
	LogHelper.info("Init done");
}

@Mod.EventHandler
public void postInit(FMLPostInitializationEvent event){
	LogHelper.info("Post-init starting, scanning environment");
	CheckAround.verifyEnvironmentSanity();
	LogHelper.info("Environment scanned");
	LogHelper.info("Post-init done");
}
}

 

You never use that class outside of it. Therefore it never gets loaded, and never instantiate the INSTANCE field, and never registers your BucketHandler as an event handler. Easiest solution in my opinion: move the registration outside of the class, e.g. in init in your main mod file.

And also material is set to material.water - probably you should creat an own material for it?