I don't know about the rest of Central America, but I didn't
have much trouble getting parts in Guadalajara for my '72 bus
engine (stock 1700 w/single Weber and a new Mexican muffler).
Repairs were never fast, but nothing in Mexico is.  Also, be
sure to doublecheck repairs (and maybe even supervise a little
after making friends with the mechanics so as not to be
insulting).  After an engine R&R, the mechanics routed my fuel
line over sharp engine tin near a heat exchanger.  Definitely
would have resulted in total destruction had I not caught it
before driving off.

My experience was that prices for good quality oil in the Mega
Mercado or Sam's Club or Wal Mart were reasonable (~5
pesos/quart = US$1.50) in 1994, and *more* than reasonable
(maybe 6 pesos, which was then $1 or so) after the peso dropped
in Dec/Jan 95.  Valvoline was easily available, and stores
usually also carried Chevron and a couple of other decent
brands.

I was initially really careful about filtering my fuel (by hand,
even), then eventually relaxed enough to let my inline filter
take care of the job.  I did keep up the octane booster and lead
substitute regimen that I had started once I hit the border,
especially if it was a hot day or I was going to be driving
hard.  The fact that Guadalajara is at 6000 feet probably helped
keep it running reasonably rich, and I probably didn't need to
worry about the octane as much as I did.

Dennis