So long as you use an oil which meets or exceeds the standards and designations called for in the manual, Harley-Davidson cannot void your warranty, under the Magnusson-Moss Act (MMA). The relevant text of the MMA reads:
"(c) No warrantor of a consumer product may condition his written, or implied,
warranty of such product on the consumer's using, in connection with such product,
any article or service (other than article or service provided without charge
under the terms of warranty) which is identified by brand, trade, or corporate name;
except that the prohibition of this subsection may be waived by the Commission
if -- (1) the warrantor satisfies the Commission that the warranted product will
function properly only if the article or service so identified is used in connection
with the warranted product, and (2) the Commission finds that such a waiver is in
the public interest." (42 U.S.C. 2302(C))
Essentially, the MMA specifically prevents manufacturers of OEM vehicles from requiring a consumer to use an OEM part, or fluid, in order to maintain warranty coverage unless that part or fluid is provide FREE OF CHARGE. (Ever get any free oil from Harley? I never did...) The law is designed to prevent manufacturers from by holding customers hostage by charging obscene prices for parts or fluids which are otherwise commonly available from the aftermarket, assuming a set of minimum standard are met. The key here is that Mobil 1, Amsoil, etc. lubricants meet or exceeds the minimum standars set by the OEM manufacturer. Seems there was a thread on oil comparisons amongst the various brands.