Several reasons, most are personal and based on our local ag applicator's
recommendations. 4 A&P A/I brothers that grew up in/on/under Lycoming
engines, mostly 540's. They've pushed them beyond what you or I will ever
do and in conditions much worse. They swear that when run hard and hot, the
multi weight oils will still give you cam problems, even with the additives.
Right or wrong, I don't know, I'm just going on what their experience has
been and using straight weights they get some longggg hours out of the
540's.

I do know, that in our tightly cowled Aztec, that 15W50 will produce more
carbon in the summer than W100. Even with W100, we often have to fly with
the cowl flaps 1/2 or even full open on hot days. The W100 doesn't produce
the carbon in the filter like the 15W50 while maintaining the same temps.

W100 is less than 1/2 the cost of 15W50, especially if you by it by the 55
gal drum. We change our oil at 33 hours and using 6 gallons at a crack, it
adds up to some $$$.

We switch to 15W50 around Thanksgiving and run it until around Easter.
Based in Wisconsin, we normally fly towards warmer weather in the winter, so
the multi weight keeps us covered on the temperature variations. We don't
want to take that mid-winter flight down south and be stuck with say, W80
and end up not being able to keep things cool enough. In the summer, the
W100 handles any temps we run into.

Our newest engine (about 600 hours old) is a Pen Yan overhaul. The front
cover of the log book has a sticker from them saying that all warrantees are
void if any multi weight oil has been used in the engine. The previous
owner used W80 and then switched to multi weights over the time he owned it.
So I guess he voided the warranty before we bought it, for what ever that's
worth.

It's the best compromise that we've come up with. Like I said, most of our
reasons are personal.

http://www.avweb.com/news/maint/182909-1.html

Is an old (1995) avweb article, maybe there is something newer.


Jim