I have a 2006 CRD Liberty. The truck is in Belize and not used a great deal in the summer when I'm in the states. A friend uses it a little for short trips. I changed the oil and filter at the end of April..What is a good period between oil changes. I've used the three month 3000 miles guide for other cars/trucks but have read that with the new oils 7500 or 1-years works.
any comments.

I would recomend at least every 5k or 1 year whichever comes first...

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Supporting Vendor and Moderator of LOST
05 Jeep Liberty CRD Limited
Ironman Springs/Bilstein/Shocks
Yeti StgIV Hot Tune
Week's BatteryTray
No FCV/EGR
Samcos/ProVent
SunCoast/Transgo
Carter Intank-pmp
2mic.Sec.Fuel Filter
Flowmaster/NO CAT
V6Airbox/noVH
GM11 Bld.fan/HDClutch
IronrockArms/wwdieselMount
98 Dodge Cummins 24 Valve

3 month/3,000 miles is only recommended by oil change places that want lots more of your business and has been outdated for decades with the possible exception of "vintage" vehicles which an 06 KJ is not. If you are using a quality diesel rated full synthetic oil (ex. Shell Rotella T-6 5w-40) every 6,000 miles or once a year which ever comes first works fine. If you/your friend was only doing short drives for a lot of the year in cold weather I might consider a shorter interval but it's not exactly cold in Belize so I would presume even short drives get up to operating temp so my recommendation stands.

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Sold to LOST member my 05 Ltd, GDE Stg II turbo + TCM tune, SunCoast TC w. Transgo kit, Steiger window regulators, Samcos, Fumoto valve, 2nd gen filter head with Lub. Spec. bleeder, Hayden clutch & 11 blade fan, inverted spare, P-1 battery, BF Goodrich Long Trail TAs, Etecno1 glow plugs, timing belt at 50K miles/8 yrs

So I've recently sent a couple of samples to blackstone labs, one was 5k and the other 6.6k, and both came back with good viscosity and tbn numbers. The only thing is I have a 2 micron filter bypass filter to help keep spot levels down. I use rotella t6 and mobil1 turbo diesel truck, whichever is cheaper.

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2006 CRD - GTB2056 turbo by Dieselguy86, Eco Trans Tune, Lift Pump, Week's, HDS Tstat, Racor Filter, ARP's, OME 790's+Top Plate, JBA 2.5", JBA UCA, Moab's+265/75R16, ARB Bull Bar, 4.10's, TrueTracs