I have been using Amsoil 15W40 heavy duty synthetic and it eliminated a noisy lifter on my '87 300D turbo but I have to add that I changed out all of the lifters to new ones when I had the valve guides replaced and the head resurfaced so that could be a factor.
I get Amsoil in 2.5 Gal jugs, it makes filling the beast easy.
http://www.amsoil.com/storefront/ame.aspx
I have a Amsoil dealer that gives a discount over what others charge for it.
We have no WalMarts around here, that is fine w/ me I am not happy supporting that organization.