Too Much Synthetic Oil

Adding too much to the gas creates an insulative effect on the piston top which creates too much wash area which could mistakenly be interpreted as "too rich". Too much oil doesn't cause any real problems but it will allow fouled plugs if the needle jetting is too rich, and it actually decreases power since synthetic oil burns too slow. The order of best to worst oils for adding to power are castor, mineral, semi-synthetic, synthetic. [read more] But synthetic is the oil of choice for high RPM high performance engines due to its high flash point which keeps it on the cylinder wall instead of evaporating off. Maybe this "excessive wash" phenomenon is the surest sign of too much oil being added.
Oils that are truly fully synthetic or "close enough" are Bel-Ray MC-1, Castrol A747, Klotz R-50, Motul 800, Repsol Off Road, Repsol Moto Racing, Silkolene Competition 2, Silkolene Pro 2, Honda HP2, ELF HTX 976+, Bel-Ray H1-R, Maxima Kart Grand Prix, Maxima Formula K-2, Castrol Power1 XR 77. The synthetic/castor blends can be included in this category too.

Reasons To Not Use Too Much Synthetic Oil:
1) synthetics burn too slow and so add little to no combustion force, unlike semi-synthetics and mineral oil which do increase power
2) the purpose of engine oil is to protect the engine and their main factor of protection is their viscosity, the more viscosity the more protection. So Motul 800 which has twice the viscosity as Yamalube at 150C can be used at twice the ratio.
3) synthetics cost more because less is required for the same protection but actually are often more cost effective when used at the correct ratio.
4) fuel/oil ratios have no "magic number" for all oils in all bikes. Here's two examples: Motul recommends their 800 Off Road synthetic be used at 50:1. Bel-Ray's rep emailed me that he recommends their H1-R synthetic be used at 40:1 for 125cc motocross.
5) too much oil adds to any existing spooge

Exception: I would only make an exception for high compression engines which get so hot that overly rich jetting is needed to cool the piston, but that reduces engine power. In that case too much synthetic oil would reduce piston heat by coating it excessively and not require overly rich jetting.

Click here to read all up on the subject of 2 stroke engine oils.