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bucha questions about oil (and my attempts at answers)

>>I wrote:

>>I use 15/50 summer, 10/30 winter, mixed fall/spring
>>(mixing, BTW, results in an averaged weight per the API).

>Do you literally mix the oils?  What does average weight per API mean?
>Maybee I'm just stupid, but I don't have the slightest clue.
>I have used Mobil1 synthetic 15W50 in the past, but thought it was a 
>little too much of an oil for me.  It's a racing oil, and expensive, 
>although it is the best.
>I have been using Castrol 5W50 Syntec motor oil for a while now 
>and it seems to work just fine.  I use it during the winters also, 
>is this bad?  The winters here are mild, but the summers 
>can be brutal.
>You also said "5/50 weights are a double edged sword."
>What did you mean by this?  Would you care to explain.
>The previous owner used 10W30 turbo V motor oil--I think Valvoline, 
>is this a good oil?  He changed it every 3k mi.  I made the switch 
>to synthetic b/c my mechanic recommended it.  I change my oil 
>about every 4-4.5k mi.  Why is a thicker motor oil better?  
>Can it absorb more heat--coat parts beter?  Should I switch 
>back to Mobil1 15W50?  What about winters, even though 
>they are mild here?
>Stupid about oils,
>Dan Hussey
>'88 5kTQ

1.) Yes, I literally just mix the oils.  The
American Petroleum Institute (API) says that
you simply average the weights.  e.g.: equal
parts 10-30 and 20-40 yeild 15W-35.  It's really
that easy!

2.) You state that Mobil 1 is "too much oil"
and "the best".  If you feel that way, why not
use it?  It costs no more than Castrol Syntec.

3.) re: "Double edged sword":, I also wrote that
viscosity extenders increase sludge production,
also per the API.  That's the answer.  Buying
an oil with a wider viscosity spread than you
need (eg 2W-100) increases buildup of sludge,
all other things equal (which, of course, they
rarely are).

4.) Valvoline TurboV is a well-regarded oil,
particularly for turbo engines.  It was the
first conventional oil to withstand the heat
of turbo bearings during heat soak (hot time
immediately after they stop running) without

5.) If any of this arcania is confusing - and
I would guess it is - then I go back to my earlier
summary to use any old SH oil, change it often,
and take it easy when the oil and motor are cold!