The varnish part I had never heard of. If you were using a non detergent oil and switched to a detergent oil, synthetic or not, it could do that.
With the seals, if your engine is already leaking, it could get worse with synthetic. I dont really know why, but have experienced it. Swapped back to Dino juice and it quit leaving puddles.
If it is an older engine, the seals are designed to swell slightly with conventional oil. Early synthetics would not do this, so they leaked. Synth oil now is supposed to have additives to correct this, and modern seals are better too.
As far as which oil to break in an engine with, it depends on who you talk to.
I would run SAE30 or 10W40 at summertime temps. Probably stick with 10w40 the rest of the time, or 5-10w30 on a fresh engine.
