I use marine grade bearing grease as appropriate for the M1 and M1A rifles and on the SIG 556R bolt/rails (gas piston and stays very clean in the receivers vs. the DI AR15/M16.)
As addicted as I am to the idea that a few minutes spent cleaning/lubing increase the likelihood of less issues later I hate to use grease on an AR15. Just makes it harder to wipe clean every time it gets shot and harder to keep clean fingers. If I had a gas piston AR15 (almost bought a SIG516 a couple times) I'd probably try grease on the bolt lugs and the high spots on the bolt that ride against the inside of the upper receiver.
Yeah, marine bearing grease is super thick and sticky -- and not at all like Slip2000's grease, which is thinner than petroleum jelly and thin enough to almost become a liquid just from the heat of one's hands. Actually, what I'd guess I'd most compare it to is coconut oil in high 70 degrees -- on the verge of transitioning from a solid to a liquid and quite slippery but before it liquifies.
Slip is a dual lubricant/penetrant, so the thicker formulations EWL30 and grease are really needed if you want the places you lube to stay wet.
Slip isn't the best rust protectant, and especially older blued guns intended for sporting afield need greater protection than Slip can provide IMO, but overall it's good to have an option that's not only effective for most of my needs but also has a neutral safety profile... And it also washes out in warm water as well vs. some of the automotive and especially marine greases -- I generally use Amsoil for my auto grease needs FWIW.