On Mon, 21 Nov 2005 18:11:10 -0500, Ron Natalie
wrote:

The reason you are told to run in a single tank mode abouve
5000' is to get increased fuel flow through whatever line you
are using to avoid vapor lock. It's a relative new (to the
design) suggestion.

This appears to be model-specific. There is no such recommendation,
placard, or instruction in the aircraft or POH for the R and S models
that I have flown/have access to, and except to actively balance the
tanks I don't operate with the selector out of the 'both' position,
regardless of altitude.

They've made some changes in the system with the newest models, the
newer (R,S) models have vented caps on both tanks, as well as the
normal one on the left strut (in addition to some minor things for the
injected engine).