oil availability, mixing, and cleaning

[foxxinabox at wideopenwest.com (Foxx (in a box))]

   i meant to change the oil in my '85 tonight, however, i found i was 1 
quart short. so i drive to autozone only to find that they were sticking 
20w-50 where my 15w-40 was supposed to go. okay, so i drive to another 
store only to find they have no 15w-40 either. fine. i do some running 
around and try a third store. no 15w-40. wtf? i've bought it at two of 
those three places before. it is a weight that they're slowly phasing 
out? i even considered mixing brands but i can find 15w-40 at all. all i 
need is one lousy quart. and i was going to stay with 10w-40 from now on 
anyway.
   this brings us to my second question: i read somewhere that oil of 
different <blank> can be mixed. is that <blank> a) weight (eg - 10w-30 & 
10w-40) or b) viscosity (eg - 10w-40 & 15w-40)? i believe it's the 
latter but i want to make sure.
   lastly, the gf's vr6 passat has tapping lifters on start up. i would 
like to quiet it them down and have used 10w-30 to no avail. i know that 
there are commercial oil passage cleaners on the market (marvel mystery 
oil comes to mind), but i was wondering if they even work and/or will 
solve the tapping lifter problem.
   any help or insight regarding any of the above topics would be 
appreciated. thanx.

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