Ok guys, I require your expertise here. I have been reading some articles where they talk about "using drops of oil as a filter". What do they mean by making a filter? And do you smear it going a certain direction? How much do you use? Please help.........

I use filters a lot. My interpretation of it, is when you have painted base colour, and then detail painted all the extra details, spraying filters over them, ties them all together.
I basecoat my model, then add a lighter colour and spray the centre of panels as a pre-shading. Then I add a dark wash to panel lines and around details to give depth. Then after a light drybrushing, all the details stand out, but its not very realistic. Over this I spray filters to blend all these "base" colours together. The filter is very thin ... using oils or enamels. 95/5 thinners/paint or thinner. I use earthy coloured filters around the running gear and lower hull which ties them together and adds to the weathered effect. Higher up I use a base colour filter or raw umber. When all the tools and stowage are painted, I spray another filter to tie these together with the tank, which also blends in the decals naturally.
Some people use loads of filters to vary the shades, as in reality a vehicle has different shades of its base colour after wear, weather and conditions.
In the two images below, you can see the effect from base paint, to when washes, dry brushes and filters have been applied. What I like about it is, that all the detail stands out, but is more natural.

I believe Mig Jimeniz introduced the term 'filter' to the modeling lexicon. What he is talking about are very thin washes applied to subtly alter the tint of a surface. What separates this from a traditional wash is that it is not applied so as to darken recesses. So, rather than removing pigments left on broad surfaces from a traditional wash, they are to remain there. Plus, filters are usually considerably more thinned than traditional washes. The colors used are generally complimentary, rather than contrasting. For example, on an olive drab surface, a yellowish brown filter may be used to impart a faded or dusty look. Finally, filters are usually applied in layers. Highly skilled masters of the process, like Mig himself, may use many layers of filters of slightly different tints to acheive a certain look. Basically, when using filters, the first layers should barely be noticeable. Hope this helps!