Cheers for the info about what's in lubes and for alternatives, Graeme and Ghost! I generally don't use cosmetic products. I only used shampoo while at uni because my Mum bought it for me. I think my hair is actually softer now than it was when I didn't use shampoo. Anyways, I'm kinda like you Graeme and just steer away from anything that contains ingredients with a high number of syllables in its name. I don't know if it would have much long term effects, but I'd rather keep away seeing as though I'm in my ripe years. Also, I wouldn't want there to be faulty genes in my sperm which'll result in my child having mental and physical difficulties. I'm sure there'd be a greater effect of chemicals screwing up the genetic code in sperm. Sort of like a chain reaction, there. :/
Anyways, leave the poor guy alone! He wanted advice on anal toys, not lube.
But... I need to say this... perhaps the reason why you're using a lot of oil is because it's absorbed or whatever too quickly and you have to reapply..? I don't know. I guess it doesn't matter because you're happy with what you're using anyway, so I'll leave you alone with going on about lube, also. 
Just a quick add-on: 6 inches doesn't seem that bad... I went for a 7 inch dildo, which I'm having some problems with. I suppose that if you shaved off an inch or an extra half, you'd be fine, then you can work up to the 6 inch. It is very much hit an miss. When concerned with sex toys, it can be a very fussy thing with length...