This is from Sylk's FAQ's

Sylk has a pH balance of 4.5 - 4.7, similar to the normal vaginal environment, i.e. slightly acidic. This is deliberate to keep the vagina healthy and not alter the balance. Sylk is not intended as a spermicide however, its pH balance (like most other lubricants) can affect motility, the movement of sperm and therefore should not be used if you're trying to conceive. When ovulation occurs, the pH balance in the vagina changes to around 7 which is a neutral pH state when pregnancy can occur.
I think I remember them recommending pre-seed for conception (which is not helpful to you now i know).