Lets get this sorted out for once and all, in the old days in the UK painting and decorating was a trade, with the advent of DIY it became a non trade, paint suppliers made paints that you could just use straight away, dulux was a big part of this, open the can and start painting, mention thinning down dulux gloss and you would probably be hanged, but we aint in England now boyo's.
The majority of Thai paints are based on old chemical formulas, these need to be thinned down, if they are emulsion paints they need to be thinned with water, read the bucket, it will tell you how much thinning they need, personly first coat I like loads of water in the paint as it dries straight away, second coat a nice thick one and carry on from there.
Oil based paints need thinners to thin them out, these aint some magical dulux paint that covers everything in one coat, for a nice job reckon on 4 coats, not so nice 3 coats, but remember, if your doing a nice piece of furniture in gloss, your probably going to do 8 coats with sanding down in between, all comes down to how good you want the finish, and every single one of them will have thinners mixed in with the gloss.