What is the best lubricant to use for a strap on?

(she is and i am biual) So, what would be the best lubricant to buy from walmart. Okay, So the strap-on is one with the vibrating tip. it has 5 different levels of vibration..Therefor only the tip of it is hard. It was kind of Weird when she tried to put it in because the shaft part of it kept bending. It kind of turned me off. (The strap is the one guys can stick there in) females can also use it. So what is the best lube to use so that it will go in easy. Or what can we put in it to make it harder.