been reading a lot about the GTL penns plat but have yet seen a HTHL spec. other specs listed by penns look great. started using the 10-30 in my aunts 99 jeep, got the brother-in-law to spend a little more for it over semi syn, he fixes things when they fall off or do not work at all!!! impressed by the excellent Noack of the 10-30, a spec that can't be easily pumped up like VI or pour points where extra short lived additives make the spec for "new" oil look good, down the road these specs can change quickly. not a fan of the watery x-20 oils = a 1/2 mpg but more wear in the long run!!