As you simply get larger, you run into the problem that you cannot factor the creme (middle of the oreo cookie). You can force the factorization to smaller numbers by proper selection of the form. For example, by choosing your (6+30t) to be 6*(1+5x)^{5}, you can reduce the size of the remaining composite by a factor of 5. If you figure that you can probably factor 100 digit numbers, you can look for 500 digit oreos of the form 1997*6*(1+5t)^{5}, where (1+5t) is 100 digits. It took a few hours to find a 505 digit oreo of this form:
2 x 3 x 7^{5} x 601^{5} x 1697^{5} x 1997 x 2221331321127842521176503^{5} x 170419913749602947794168163^{5} x 4046509251675607979406939228671806723909451^{5}
Going much larger than this, your problem will be finding candidates. You will probably need a better search method.
William
