British Citizenship and US conviction

[BravoMav]

I applied for and received ILR in 2013. In the application I was advised by an immigration firm that I did not have to divulge a drunk driving conviction in the US because it was over 25 years old, 1987/1988. Now I would like to apply for British citizenship and I'm very worried I was given bad advice and should have divulged the conviction, the lack of which I think may disqualify me for citizenship and potentially effect my ILR status. From the citizenship application it appears I definitely have to reference the conviction even though it is 30 years old, which I have no problem with, but I'm worried about the fact that I didn't reference it on my ILR application. Lastly, it's been almost 10 years but I'm relatively confident I was required to disclose on my 5 year work VISA. I am not hiding conviction, I pleaded "no contest" to a basic DUI in the US

Any advice or guidance would be appreciated.