"Dave" wrote in message ...

"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

why not just change the whole lot and do the job properly?

Shell says you can mix grades of AeroShell as long as both grades
are allowed for your engine.