SLR lens maintenance advice please!

Hi, does anyone know what type of grease is suitable to use on an SLR lens? I have several lenses but due to use they have started making gritty rumbling noises as I'm using them (twisting the barrel for zoom/focus), it's just the bearings inside making the noise I expect, but I'd like to get a smoother action on them and don't want to put any old grease in there in case it messes up the lens, anybody have any ideas? If someone knows how to get bits of dust and stuff out from inside the lens that would be great to know too!

Thanks in advance
Lee

I would be very carfull about taking lenses apart myself, but would send them to a servicing company, just because thay are expensive. I have never had problems with scraping noises in barings, but grit between focusing or zoom rings on the lense. a cost of a service is always going to be cheaper than a replacement lense.

Sam