YES, you can use for word “lifetime” in the sales letter, but we advise that you make it very clear to customers what lifetime means in your offer especially for warranty and guarantee part.
For example, if “lifetime” in your offer means the life of your product, we advice to make it very clear to the consumer what it means and what is the warranty / guarantee consumer get.
You must always follow all FTC guidelines.
“The Guides advise that to avoid confusing consumers about the duration of a "lifetime" warranty or guarantee, ads should tell consumers which "life" measures the warranty's duration In that way, consumers will know which meaning of the term "lifetime" you intend."
Here is a link to a related part of the FTC website:
This text is for informational purposes only and does not provide legal advice.