Yep under the grill if put back in quick enough they are still hot as in a warm engine this helps the fire up as it stops the plugs getting flooded with fuel.
As for the cruddy film this is oil and carbon due to lack of regular servicing oil acts as a sponge and soaks up carbon ect same as the filter if not changed regular enough will leave a deposit behind Ive seen the valley almost full with this on more than one occasion engine may be a donkey but surely its worth firing up first? if it runs OK drain the oil and refill with 50/50 oil and parafin (kerosene) run until warm drain (thoroughly) and repeat then drain refill and run for 15 mins with some cheapo oil drain refilll and change filter with regular oil will be nice and clean then. Takes a while but it works better than flushing oil and its cheaper. Even then if you freshen it up with brgs, rings, pump lifters if req (dont forget to put cam lube on them and run them in) at least you know its good better the devil you know and all that