I'll add another vote toward ordinary oil, not energy conserving. I reckon we must put around 10,000 miles on our poor rigs each year, try to change oil once a month and they do just fine. I have no idea how many miles the old '98 TRX 300 has but it must be a pile. Engine's never been opened either and it runs real well. Excessive worry about oil and not enough and/or improper driving is way harder on the engine than any buying a certain label. Keep the engine speed up enough to provide adequate centrifugal power for good clutch engagements and they'll last a long, long time.