I have been looking at the online X-Wedge owners manual, a bit more detailed than the workshop manual in places. It says this about the oil filters:
"The S&S® X-Wedge® engine requires a spin-on filter with an anti-drain back valve, rated at 10 microns. The S&S oil filter part numbers are 31- 4104 for Chrome and 31-4103 for Black. Filters for 1999-later Harley- Davidson® big twins (#63798-99A and 63731-99A) are also acceptable."
My M3W doesn't suffer from wet sumping and I use K&N oil filters, I assume they have an anti-drain back valve? In reply to Farmer Giles, the other S&S forums seem to think that the problem relates to the oil pump as well.
It also says:
"S&S recommends synthetic engine oil such as S&S Premium Synthetic 20W50. However a premium petroleum based engine oil such as S&S Heavy Duty 20W50 is acceptable"
