Hi and welcome.
Yes the RFS and the RC4 engines only hold about
a liter of oil in them. Typically you can just drain the oil and refill, then
change the filters every other drain (I did on my '03 and got 12k miles before
doing the top end). It's a given that you must use good synthetic oil.
I really can't think of any bike honestly that can go from MX track and be
good on the road except for short little stretches.
I had the same dilemma and bought an LC4 for my dual sport use, and the Berg for single track and more technical riding.
Hope this helped
__________________
2011 Husaberg TE 300
2002 640 LC4e
Past Rides
'03 KTM 450 EXC (SOLD)
'01 Honda XR650R (SOLD)
'98 Honda XR600R (SOLD)
'95 KTM 620 RXC (SOLD)
'88 Yamaha TT600 (SOLD)
'86 Yamaha XT 350 (SOLD)
'84 Yamaha IT465 (STOLEN)
'83 Kawasaki KDX 450 (SOLD)
'83 Honda CB1000 Custom (SOLD)
'82 Kawasaki KDX 400 (SOLD)
'77 Yamaha DT200 (SOLD)
'70 Honda CT70 (SOLD)