Re: 5 yr. old OMC GelSeal
When I was in industry, we had strictly monitored date codes on a lot of products (of the sort) that we used for assembling our products. We spent a lot of time, money, and generated a lot of waste for the sake of insuring the products we used met the manufacturer's specifications. But that is what the (our) customer wanted and paid for so that's what he got.
I have never gone into a retail store and observed a date code, nor a date of manufacture on any product of the sort.
I have used products of the sort that have been in my shop for years; some opened and some not; some survived the test of time, some didn't. I give tubes the squeeze test prior to intended use and if they squeeze, there must be some life left.
Other thing was that we always used a primer prior to installing products of sort. I have never seen primer offered at local retailers.
So I guess it just depends and like Mr. Hadley said, if it holds, it must be a-workin!
My 2c.
Mark