The instances of motors developing leaks after switching to a synthetic oil are pretty old, not the problem now as when synthetics first came out.
What happened was on motors that had many miles (way more than 28,000), were run on older dino oil that didn't have the detergents that modern oils do. This allowed sludge to build up, and this sludge is what was actually keeping the motor from leaking. Then when they switched to synthetic oil with a better detergent package, it cleaned out the sludge and thus showed evidence of bad rocker box gaskets that the sludge was masking.
Even today's dino oil has a better detergent additive package than the dino oils used when synthetic oils first came to market.
'05 Carbed Night Train
111 rwhp, 112 rwtq
Best 1/4 ET 12.557s
Best 1/4 MPH 108.59