"Ben Jackson" wrote in message
news:CQkJb.261117$_M.1211945@attbi_s54...
Let's say you're at a point where you need to add a quart of oil.
You've decided you would like to finish this oil change interval at
a lighter weight (eg you put in AeroShell W100, SAE-50, and now it's
freaking cold). Can you add a quart of a (substantially?) lighter
weight oil (presumably in the same 'series')?

--
Ben Jackson

http://www.ben.com/

Aeroshell says it is just fine to do so:

"All approved SAE J-1899 (former MIL-L-22851) and SAE J-1966 (former
MIL-L-6082) AeroShell oils are compatible."