Hi guys. Here is a mysterious problem. several of us are experiencing gas in our crankcases, visible on checking the oil level via the dipstick.
There are two common factors between us.
1. we all buy our gas from the same place (regular gas)
2. All the engines are being serviced by the same mechanic.

Curiously the two guys who use super grade gas have had no problem.
The engines are mainly (but not only Yamaha 50s or 60s)
anyone any idea what the problem is?